Differential Equations in Pumas

Author

David Widmann

1 Introduction

Pumas automatically chooses a differential equation solver that is suitable for the simulation or estimation of the dynamical system of the NLME (Nonlinear Mixed Effects) model at hand. This default solver is the preferred choice and optimized for most users and use cases. Nevertheless, in some cases the performance-accuracy trade-off can be improved by adjusting the tolerances, or possibly even the algorithm, of the differential equation solver.

In this tutorial, the Warfarin PK/PD model is used to demonstrate how to configure the differential equation solver.

2 Learning Goals

  • Observe the utility of the @vars block of a Pumas model with respect to storing dynamic variables associated with differential equations
  • Understand the main differences between common differential equation solvers for nonlinear dynamical systems
  • Learn how to adjust the algorithm and the tolerances of the differential equation solver

3 Warfarin PK/PD Model

We return to the Warfarin PK/PD model. Its dynamical system consists of three states, \(\operatorname{Depot}\), \(\operatorname{Central}\), and \(\operatorname{Turnover}\), whose dynamics are governed by the ordinary differential equations:

\[ \begin{aligned} \operatorname{Depot}'(t) &= - \operatorname{Ka} \operatorname{Depot}(t),\\ \operatorname{Central}'(t) &= \operatorname{Ka} \operatorname{Depot}(t) - \frac{\operatorname{CL}}{\operatorname{Vc}} \operatorname{Central}(t),\\ \operatorname{Turnover}'(t) &= \operatorname{rin} (1 + \operatorname{emax} \frac{\operatorname{Central}(t) / \operatorname{Vc}}{\operatorname{c50} + \operatorname{Central}(t)/\operatorname{Vc}}) - \operatorname{kout} \operatorname{Turnover}(t) \end{aligned} \]

with PK parameters \(\operatorname{Ka}\) (absorption rate), \(\operatorname{CL}\) (clearance), and \(\operatorname{Vc}\) (volume of distribution) and PD parameters \(\operatorname{rin}\), \(\operatorname{emax}\), \(\operatorname{c50}\), and \(\operatorname{kout}\).

The dynamical system can be written more concisely by introducing auxiliary variables for repeated expressions:

\[ \begin{aligned} \operatorname{Depot}'(t) &= -\operatorname{ratein}(t),\\ \operatorname{Central}'(t) &= \operatorname{ratein}(t) - \operatorname{CL} \operatorname{cp}(t),\\ \operatorname{Turnover}'(t) &= \operatorname{rin} \operatorname{pd}(t) - \operatorname{kout} \operatorname{Turnover}(t) \end{aligned} \]

with influx rate \(\operatorname{ratein}(t) := \operatorname{Ka} \operatorname{Depot}(t)\), concentration \(\operatorname{cp}(t) := \operatorname{Central}(t) / \operatorname{Vc}\), and \(\operatorname{pd}(t) := 1 + \operatorname{emax} \frac{\operatorname{cp}(t)}{\operatorname{c50} + \operatorname{cp}(t)}\).

4 Auxiliary Variables in @vars

In Pumas, dynamical systems are defined in the @dynamics block inside of the @model definition. For instance, the dynamical system of the Warfarin PK/PD model can be implemented as follows:

warfarin_pkpd_model = @model begin
    ...

    @dynamics begin
        Depot' = -Ka * Depot
        Central' = Ka * Depot - CL / Vc * Central
        Turnover' =
            rin * (1 + emax * (Central / Vc) / (cp50 + Central / Vc)) - kout * Turnover
    end

    ...
end

The same concise rewriting can be applied in a Pumas @model by defining auxiliary variables (“aliases”) in the @vars block:

warfarin_pkpd_model = @model begin
    ...

    @vars begin
        cp := Central / Vc
        ratein := Ka * Depot
        pd := 1 + emax * cp / (c50 + cp)
    end

    @dynamics begin
        Depot' = -ratein
        Central' = ratein - CL * cp
        Turnover' = rin * pd - kout * Turnover
    end

    ...
end
Tip

The walrus operator (:=) ensures that the aliases do not show up in the simulation output of the model. However, if you would like to access an alias in the simulation output, you should define the alias with =. For instance, if you want to obtain concentration cp as part of the simulation output, you can change the @vars block to

@vars begin
    cp = Central / Vc
    ratein := Ka * Depot
    pd := 1 + emax * cp / (c50 + cp)
end

5 Differential Equation Solvers

The differential equation in the Warfarin model is non-linear, as detected by Pumas (“Dynamical system type: Nonlinear ODE”):

using Pumas

warfarin_pkpd_model = @model begin
    @param begin
        # PK parameters
        """
        Clearance (L/h/70kg)
        """
        pop_CL ∈ RealDomain(lower = 0.0, init = 0.134)
        """
        Central Volume L/70kg
        """
        pop_V ∈ RealDomain(lower = 0.0, init = 8.11)
        """
        Absorption time (h)
        """
        pop_tabs ∈ RealDomain(lower = 0.0, init = 0.523)
        """
        Lag time (h)
        """
        pop_lag ∈ RealDomain(lower = 0.0, init = 0.1)
        # PD parameters
        """
        Baseline
        """
        pop_e0 ∈ RealDomain(lower = 0.0, init = 100.0)
        """
        Emax
        """
        pop_emax ∈ RealDomain(init = -1.0)
        """
        EC50
        """
        pop_c50 ∈ RealDomain(lower = 0.0, init = 1.0)
        """
        Turnover
        """
        pop_tover ∈ RealDomain(lower = 0.0, init = 14.0)
        # Inter-individual variability
        """
          - ΩCL
          - ΩVc
          - ΩTabs
        """
        pk_Ω ∈ PDiagDomain([0.01, 0.01, 0.01])
        """
          - Ωe0
          - Ωemax
          - Ωec50
          - Ωturn
        """
        pd_Ω ∈ PDiagDomain([0.01, 0.01, 0.01, 0.01])
        # Residual variability
        """
        Proportional residual error for drug concentration
        """
        σ_prop ∈ RealDomain(lower = 0.0, init = 0.00752)
        """
        Additive residual error for drug concentration (mg/L)
        """
        σ_add ∈ RealDomain(lower = 0.0, init = 0.0661)
        """
        Additive error for PCA
        """
        σ_fx ∈ RealDomain(lower = 0.0, init = 0.01)
    end

    @random begin
        # mean = 0, covariance = pk_Ω
        pk_η ~ MvNormal(pk_Ω)
        # mean = 0, covariance = pd_Ω
        pd_η ~ MvNormal(pd_Ω)
    end

    @covariates FSZV FSZCL

    @pre begin
        # PK
        CL = FSZCL * pop_CL * exp(pk_η[1])
        Vc = FSZV * pop_V * exp(pk_η[2])
        tabs = pop_tabs * exp(pk_η[3])
        Ka = log(2) / tabs
        # PD
        e0 = pop_e0 * exp(pd_η[1])
        emax = pop_emax * exp(pd_η[2])
        c50 = pop_c50 * exp(pd_η[3])
        tover = pop_tover * exp(pd_η[4])
        kout = log(2) / tover
        rin = e0 * kout
        time = t
    end

    @dosecontrol begin
        lags = (Depot = pop_lag,)
    end

    @init begin
        Turnover = e0
    end

    # aliases for use in @dynamics and @derived
    @vars begin
        cp := Central / Vc
        ratein := Ka * Depot
        pd := 1 + emax * cp / (c50 + cp)
    end

    @dynamics begin
        Depot' = -ratein
        Central' = ratein - CL * cp
        Turnover' = rin * pd - kout * Turnover
    end

    @derived begin
        """
        Warfarin Concentration (mg/L)
        """
        conc ~ @. CombinedNormal(cp, σ_add, σ_prop)
        """
        PCA
        """
        pca ~ @. Normal(Turnover, σ_fx)
    end
end
PumasModel
  Parameters: pop_CL, pop_V, pop_tabs, pop_lag, pop_e0, pop_emax, pop_c50, pop_tover, pk_Ω, pd_Ω, σ_prop, σ_add, σ_fx
  Random effects: pk_η, pd_η
  Covariates: FSZV, FSZCL
  Dynamical system variables: Depot, Central, Turnover
  Dynamical system type: Nonlinear ODE
  Derived: conc, pca
  Observed: conc, pca

Pumas approximates the solution of the differential equation with a numerical differential equation solver. Generally, one distinguishes between solvers for stiff and non-stiff differential equations.

5.1 Stiff vs. Non-Stiff Systems

A key distinction among numerical solvers is whether they are designed for stiff or non-stiff differential equations:

  • Non-Stiff Differential Equations: These systems exhibit relatively moderate changes in their variables. Standard non-stiff solvers can efficiently approximate solutions of these systems.

  • Stiff Differential Equations: These systems contain rapidly changing components alongside more slowly varying dynamics. Non-stiff solvers typically perform poorly on stiff systems, as they may require exceedingly small step sizes to maintain numerical stability. Specialized stiff solvers are therefore employed to handle the sharp gradients and large timescale differences without compromising accuracy.

6 Pumas’s Automatic Solver Selection

By default, Pumas adopts a hybrid approach with automatic stiffness detection to switch between stiff and non-stiff solvers as needed.

  • Default Solvers: Rodas5P (stiff) and Vern7 (non-stiff)
  • Tolerances: Relative tolerance \(10^{-8}\) and absolute tolerance \(10^{-12}\)
  • Rationale: These tolerances ensure high precision during both simulation and parameter estimation, which is critical for accurate exploratory and predictive modeling and matching the model’s predictions to observed data.

The default solvers and tolerances are recommended for most users in most instances. If desired, however, it is possible to adjust these settings with the diffeq_options keyword argument.

Important

Computation time decreases as tolerances are increased. However, higher tolerances come at the cost of a less strict error control, and hence generally a less accurate solution.

6.1 Adjusting the Tolerances

The absolute and relative tolerance of the solver can be specified with abstol and reltol.

7 Absolute and Relative Tolerances

When employing a numerical solver, it is necessary to specify how accurately the solution should be computed. This precision is controlled by two key parameters:

  1. Absolute Tolerance \((\text{abstol})\)

    • Interpreted as the maximum allowable error when the solution values are near zero.
    • Ensures that numerical approximations stay within a reasonable bound, preventing physically impossible outcomes (e.g., negative concentrations) or excessive drift at small scales.
    • For instance, an absolute tolerance of \(10^{-6}\) means the solver attempts to keep the absolute error below \(10^{-6}\) whenever the solution magnitude is close to zero.
  2. Relative Tolerance \((\text{reltol})\)

    • Enforces the number of correct digits throughout the simulation, effectively controlling error relative to the current scale of the solution.
    • For example, a relative tolerance of \(10^{-3}\) implies the solver aims for three correct decimal places (i.e., the solution is accurate to within 0.1% of its current magnitude).
    • As the solution grows or shrinks, the solver adjusts its time-step size and internal computations to maintain this relative accuracy.

Sometimes decreasing tolerances can help to reduce numerical problems, e.g. to keep solutions non-negative that are mathematically guaranteed to be non-negative. Additionally, the choice of tolerances can be motivated by the application of the numerical solution: For plotting a less accurate solution, and hence larger tolerances, might be tolerable, whereas typically for model fitting a more accurate solution, and hence smaller tolerances, are beneficial.

This can be demonstrated when fitting the Warfarin model with an example dataset: Optimization fails with large tolerances of 1e-3 (relative) and 1e-6 (absolute)

fit(
    warfarin_pkpd_model,
    pop,
    init_params(warfarin_pkpd_model),
    FOCE();
    diffeq_options = (; reltol = 1e-3, abstol = 1e-6),
)
[ Info: Checking the initial parameter values.
[ Info: The initial negative log-likelihood and its gradient are finite. Check passed.
Iter     Function value   Gradient norm 
     0     3.130049e+06     5.915673e+06
 * time: 0.10085296630859375
     1     5.185947e+05     8.741866e+05
 * time: 31.95722794532776
     2     3.866970e+05     6.366598e+05
 * time: 33.38027906417847
     3     1.795038e+05     2.835250e+05
 * time: 33.979835987091064
     4     9.682944e+04     1.546505e+05
 * time: 34.59925889968872
     5     4.791895e+04     6.819657e+04
 * time: 35.20427703857422
     6     2.907473e+04     3.509578e+04
 * time: 35.8135130405426
     7     1.827466e+04     1.713041e+04
 * time: 36.413214921951294
     8     1.260726e+04     9.564194e+03
 * time: 36.99838900566101
     9     9.403896e+03     8.612114e+03
 * time: 37.54739999771118
    10     7.326086e+03     7.634997e+03
 * time: 38.088762044906616
    11     5.927094e+03     6.625137e+03
 * time: 38.59825801849365
    12     4.942762e+03     5.562953e+03
 * time: 39.14595890045166
    13     4.139610e+03     4.326715e+03
 * time: 39.62843990325928
    14     3.563998e+03     3.065221e+03
 * time: 40.12851095199585
    15     3.296712e+03     2.170265e+03
 * time: 40.61025810241699
    16     3.216830e+03     1.669547e+03
 * time: 41.10696196556091
    17     3.206499e+03     1.487841e+03
 * time: 41.588062047958374
    18     3.205537e+03     1.444242e+03
 * time: 42.09421110153198
    19     3.204662e+03     1.417622e+03
 * time: 42.61991596221924
    20     3.201697e+03     1.361560e+03
 * time: 43.163373947143555
    21     3.194761e+03     1.282787e+03
 * time: 43.667803049087524
    22     3.176327e+03     1.157868e+03
 * time: 44.181999921798706
    23     3.130481e+03     9.772233e+02
 * time: 44.680757999420166
    24     3.017242e+03     7.263321e+02
 * time: 45.15919804573059
    25     2.750133e+03     4.106853e+02
 * time: 45.6402530670166
    26     2.137975e+03     2.318410e+02
 * time: 46.0722930431366
    27     1.357961e+03     1.422407e+02
 * time: 46.774665117263794
    28     1.296792e+03     2.619606e+02
 * time: 47.43133997917175
    29     1.276960e+03     1.838772e+02
 * time: 47.817694902420044
    30     1.267251e+03     1.950227e+02
 * time: 48.1839759349823
    31     1.246464e+03     2.121277e+02
 * time: 48.58478903770447
    32     1.244107e+03     1.960492e+02
 * time: 48.979257106781006
    33     1.243941e+03     1.908395e+02
 * time: 49.3779571056366
    34     1.243927e+03     1.905050e+02
 * time: 49.78055000305176
    35     1.243788e+03     1.876624e+02
 * time: 50.16678595542908
    36     1.243545e+03     1.837093e+02
 * time: 50.56037497520447
    37     1.242827e+03     1.738853e+02
 * time: 50.97185707092285
    38     1.241221e+03     1.547074e+02
 * time: 51.38754391670227
    39     1.237772e+03     1.170472e+02
 * time: 51.78935503959656
    40     1.232408e+03     5.798690e+01
 * time: 52.191704988479614
    41     1.227950e+03     4.597807e+01
 * time: 52.5912880897522
    42     1.226363e+03     5.777629e+01
 * time: 52.99941802024841
    43     1.226084e+03     5.151307e+01
 * time: 53.43212008476257
    44     1.226077e+03     4.917111e+01
 * time: 53.868854999542236
    45     1.226076e+03     4.892725e+01
 * time: 54.283987045288086
    46     1.226069e+03     4.758509e+01
 * time: 54.69982099533081
    47     1.226057e+03     4.572770e+01
 * time: 55.11691999435425
    48     1.226020e+03     4.166826e+01
 * time: 55.53787112236023
    49     1.225933e+03     3.401203e+01
 * time: 55.959665060043335
    50     1.225728e+03     2.835333e+01
 * time: 56.358500957489014
    51     1.225334e+03     2.817036e+01
 * time: 56.75296401977539
    52     1.224813e+03     3.424252e+01
 * time: 57.15521311759949
    53     1.224477e+03     4.798431e+01
 * time: 57.5795259475708
    54     1.224395e+03     4.437361e+01
 * time: 57.97052597999573
    55     1.224386e+03     3.979127e+01
 * time: 58.35424304008484
    56     1.224385e+03     3.851708e+01
 * time: 58.73781895637512
    57     1.224383e+03     3.680673e+01
 * time: 59.11666703224182
    58     1.224376e+03     3.402533e+01
 * time: 59.51831793785095
    59     1.224359e+03     3.268119e+01
 * time: 59.90696310997009
    60     1.224314e+03     3.090784e+01
 * time: 60.28792691230774
    61     1.224196e+03     2.816919e+01
 * time: 60.67724895477295
    62     1.223896e+03     2.822262e+01
 * time: 61.06475806236267
    63     1.223162e+03     3.872359e+01
 * time: 61.46628403663635
    64     1.221566e+03     7.595349e+01
 * time: 61.8562068939209
    65     1.219004e+03     9.939356e+01
 * time: 62.26878595352173
    66     1.216625e+03     7.736576e+01
 * time: 62.69918394088745
    67     1.215517e+03     7.064562e+01
 * time: 63.108803033828735
    68     1.215381e+03     6.708583e+01
 * time: 63.509095907211304
    69     1.215375e+03     6.675536e+01
 * time: 63.888076066970825
    70     1.215371e+03     6.640207e+01
 * time: 64.2688980102539
    71     1.215359e+03     6.551217e+01
 * time: 64.65693402290344
    72     1.215331e+03     6.395606e+01
 * time: 65.0343689918518
    73     1.215255e+03     6.066088e+01
 * time: 65.41432094573975
    74     1.215062e+03     5.381643e+01
 * time: 65.81467700004578
    75     1.214585e+03     3.902384e+01
 * time: 66.19644403457642
    76     1.213513e+03     5.729613e+01
 * time: 66.5833740234375
    77     1.211585e+03     7.543450e+01
 * time: 66.9708399772644
    78     1.209446e+03     1.047478e+02
 * time: 67.35754299163818
    79     1.208363e+03     1.260683e+02
 * time: 67.76032209396362
    80     1.208147e+03     1.160237e+02
 * time: 68.1766710281372
    81     1.208132e+03     1.100932e+02
 * time: 68.55112290382385
    82     1.208127e+03     1.084514e+02
 * time: 68.92857193946838
    83     1.208106e+03     1.039380e+02
 * time: 69.30905795097351
    84     1.208062e+03     9.804794e+01
 * time: 69.70384097099304
    85     1.207936e+03     8.751468e+01
 * time: 70.08549308776855
    86     1.207621e+03     7.057805e+01
 * time: 70.4692919254303
    87     1.206819e+03     6.533132e+01
 * time: 70.8565559387207
    88     1.204953e+03     5.687190e+01
 * time: 71.26108407974243
    89     1.201401e+03     5.997970e+01
 * time: 71.66377401351929
    90     1.197045e+03     8.595365e+01
 * time: 72.04711508750916
    91     1.193720e+03     5.814563e+01
 * time: 72.44187903404236
    92     1.192424e+03     4.918237e+01
 * time: 72.82445502281189
    93     1.192332e+03     5.056415e+01
 * time: 73.20462012290955
    94     1.192328e+03     5.064817e+01
 * time: 73.64135003089905
    95     1.192328e+03     5.069435e+01
 * time: 74.0106770992279
    96     1.192324e+03     5.077239e+01
 * time: 74.39533710479736
    97     1.192315e+03     5.089603e+01
 * time: 74.79334998130798
    98     1.192292e+03     5.107354e+01
 * time: 75.18372201919556
    99     1.192233e+03     5.128045e+01
 * time: 75.58056402206421
   100     1.192079e+03     5.138224e+01
 * time: 75.99807810783386
   101     1.191690e+03     5.093178e+01
 * time: 76.38790512084961
   102     1.190747e+03     6.233310e+01
 * time: 76.76628303527832
   103     1.188711e+03     8.393857e+01
 * time: 77.1613199710846
   104     1.185262e+03     8.919115e+01
 * time: 77.57585096359253
   105     1.181507e+03     6.478905e+01
 * time: 78.0280430316925
   106     1.179507e+03     6.342536e+01
 * time: 78.47319197654724
   107     1.179066e+03     5.651175e+01
 * time: 78.92499899864197
   108     1.179007e+03     5.355351e+01
 * time: 79.39091396331787
   109     1.178989e+03     5.342382e+01
 * time: 79.82625102996826
   110     1.178981e+03     5.452378e+01
 * time: 80.24451088905334
   111     1.178978e+03     5.571710e+01
 * time: 80.65409207344055
   112     1.178976e+03     5.645145e+01
 * time: 81.0625069141388
   113     1.178974e+03     5.703539e+01
 * time: 81.486407995224
   114     1.178968e+03     5.789071e+01
 * time: 81.88936305046082
   115     1.178951e+03     5.907442e+01
 * time: 82.2967050075531
   116     1.178908e+03     6.064947e+01
 * time: 82.71039199829102
   117     1.178799e+03     6.229592e+01
 * time: 83.13639211654663
   118     1.178533e+03     6.273385e+01
 * time: 83.57649397850037
   119     1.177947e+03     5.849461e+01
 * time: 83.99810194969177
   120     1.176912e+03     4.388558e+01
 * time: 84.42919611930847
   121     1.175763e+03     2.091799e+01
 * time: 84.88180804252625
   122     1.175177e+03     2.044403e+01
 * time: 85.3150749206543
   123     1.175045e+03     2.016046e+01
 * time: 85.76054501533508
   124     1.175028e+03     2.019076e+01
 * time: 86.20812606811523
   125     1.175028e+03     2.022824e+01
 * time: 86.62758994102478
   126     1.175027e+03     2.024135e+01
 * time: 87.06428790092468
   127     1.175026e+03     2.027385e+01
 * time: 87.47796702384949
   128     1.175023e+03     2.030820e+01
 * time: 87.88658905029297
   129     1.175015e+03     2.036436e+01
 * time: 88.31808805465698
   130     1.174993e+03     2.043864e+01
 * time: 88.73513007164001
   131     1.174937e+03     2.054254e+01
 * time: 89.15437889099121
   132     1.174789e+03     2.067532e+01
 * time: 89.59600496292114
   133     1.174399e+03     2.080795e+01
 * time: 90.01402711868286
   134     1.173370e+03     2.144892e+01
 * time: 90.45199704170227
   135     1.170694e+03     3.548471e+01
 * time: 90.90973806381226
   136     1.164233e+03     5.629167e+01
 * time: 91.37341809272766
   137     1.154010e+03     6.151737e+01
 * time: 91.89041304588318
   138     1.150573e+03     1.639707e+02
 * time: 92.41977190971375
   139     1.147394e+03     2.917842e+01
 * time: 93.13551592826843
   140     1.145496e+03     2.863275e+01
 * time: 93.65010809898376
   141     1.143192e+03     3.094888e+01
 * time: 94.13974595069885
   142     1.142078e+03     2.111828e+01
 * time: 94.60862302780151
   143     1.141269e+03     2.280164e+01
 * time: 95.10577297210693
   144     1.140892e+03     2.102961e+01
 * time: 95.5939519405365
   145     1.140884e+03     2.116517e+01
 * time: 96.05637502670288
   146     1.140881e+03     2.120892e+01
 * time: 96.53991889953613
   147     1.140875e+03     2.117831e+01
 * time: 97.62418508529663
   148     1.140876e+03     2.117287e+01
 * time: 98.84433007240295
   149     1.140877e+03     2.116565e+01
 * time: 100.18232703208923
   150     1.140878e+03     2.115760e+01
 * time: 101.80237102508545
   151     1.140878e+03     2.108868e+01
 * time: 102.33382201194763
   152     1.140879e+03     2.106545e+01
 * time: 102.84422707557678
   153     1.140877e+03     2.099005e+01
 * time: 103.39663600921631
   154     1.140869e+03     2.088107e+01
 * time: 103.90508198738098
   155     1.140851e+03     2.072350e+01
 * time: 104.40935897827148
   156     1.140805e+03     2.050201e+01
 * time: 104.9587140083313
   157     1.140672e+03     2.020905e+01
 * time: 105.4999930858612
   158     1.140327e+03     1.993126e+01
 * time: 106.06782007217407
   159     1.139343e+03     2.220619e+01
 * time: 106.60322999954224
   160     1.136580e+03     3.909688e+01
 * time: 107.16822910308838
   161     1.131533e+03     6.943445e+01
 * time: 107.72238206863403
   162     1.128306e+03     8.886539e+01
 * time: 108.7072479724884
   163     1.119536e+03     1.154839e+02
 * time: 109.25799202919006
   164     1.114376e+03     8.035326e+01
 * time: 110.09401106834412
   165     1.112204e+03     6.285092e+01
 * time: 110.6754059791565
   166     1.111325e+03     9.747779e+00
 * time: 111.29452800750732
   167     1.111283e+03     1.107469e+01
 * time: 112.06412696838379
   168     1.111254e+03     1.023420e+01
 * time: 112.6079330444336
   169     1.111253e+03     1.079507e+01
 * time: 113.19580912590027
   170     1.111254e+03     1.073945e+01
 * time: 114.42395901679993
   171     1.111255e+03     1.068732e+01
 * time: 123.97819805145264
   172     1.111252e+03     1.060491e+01
 * time: 133.29102110862732
┌ Warning: Terminated early due to NaN in gradient.
└ @ Optim ~/run/_work/PumasTutorials.jl/PumasTutorials.jl/custom_julia_depot/packages/Optim/z6105/src/multivariate/optimize/optimize.jl:153
┌ Warning: Terminated early due to NaN in gradient.
└ @ Optim ~/run/_work/PumasTutorials.jl/PumasTutorials.jl/custom_julia_depot/packages/Optim/z6105/src/multivariate/optimize/optimize.jl:153
┌ Warning: Terminated early due to NaN in gradient.
└ @ Optim ~/run/_work/PumasTutorials.jl/PumasTutorials.jl/custom_julia_depot/packages/Optim/z6105/src/multivariate/optimize/optimize.jl:153
   173     1.110664e+03     3.083628e+01
 * time: 175.7226219177246
   174     1.110332e+03     4.730705e+01
 * time: 176.67596793174744
┌ Warning: Terminated early due to NaN in gradient.
└ @ Optim ~/run/_work/PumasTutorials.jl/PumasTutorials.jl/custom_julia_depot/packages/Optim/z6105/src/multivariate/optimize/optimize.jl:153
┌ Warning: Terminated early due to NaN in gradient.
└ @ Optim ~/run/_work/PumasTutorials.jl/PumasTutorials.jl/custom_julia_depot/packages/Optim/z6105/src/multivariate/optimize/optimize.jl:153
┌ Warning: Terminated early due to NaN in gradient.
└ @ Optim ~/run/_work/PumasTutorials.jl/PumasTutorials.jl/custom_julia_depot/packages/Optim/z6105/src/multivariate/optimize/optimize.jl:153
┌ Warning: Terminated early due to NaN in gradient.
└ @ Optim ~/run/_work/PumasTutorials.jl/PumasTutorials.jl/custom_julia_depot/packages/Optim/z6105/src/multivariate/optimize/optimize.jl:153
┌ Warning: Terminated early due to NaN in gradient.
└ @ Optim ~/run/_work/PumasTutorials.jl/PumasTutorials.jl/custom_julia_depot/packages/Optim/z6105/src/multivariate/optimize/optimize.jl:153
   175     1.106751e+03     7.819364e+01
 * time: 200.38744401931763
   176     1.105947e+03     9.045065e+01
 * time: 201.3515899181366
   177     1.101707e+03     3.557938e+01
 * time: 202.21357893943787
   178     1.099097e+03     2.521659e+01
 * time: 207.54717803001404
   179     1.097610e+03     2.247338e+01
 * time: 209.39091897010803
   180     1.096552e+03     2.038806e+01
 * time: 210.36381793022156
   181     1.095186e+03     1.994325e+01
 * time: 211.8545560836792
   182     1.094889e+03     1.775126e+01
 * time: 212.7726559638977
   183     1.090935e+03     1.213116e+02
 * time: 213.61496806144714
   184     1.088311e+03     6.837435e+01
 * time: 214.4737319946289
   185     1.080704e+03     3.224134e+01
 * time: 215.0809919834137
   186     1.079956e+03     3.674062e+01
 * time: 216.03220200538635
   187     1.076150e+03     8.858208e+01
 * time: 216.7093780040741
   188     1.072169e+03     6.023963e+01
 * time: 217.36745405197144
   189     1.068995e+03     7.602314e+00
 * time: 218.03085803985596
   190     1.068636e+03     1.100790e+01
 * time: 218.87113404273987
   191     1.068473e+03     8.239482e+00
 * time: 219.57624697685242
   192     1.068406e+03     2.015959e+00
 * time: 220.3769989013672
   193     1.068361e+03     3.813509e+00
 * time: 221.10253810882568
   194     1.068328e+03     6.558561e+00
 * time: 222.1658890247345
   195     1.068241e+03     8.551467e+00
 * time: 223.0452959537506
   196     1.068151e+03     5.769737e+00
 * time: 223.7558469772339
   197     1.068071e+03     4.657982e+00
 * time: 224.59823489189148
   198     1.068036e+03     3.841275e+00
 * time: 225.41302394866943
   199     1.068021e+03     2.649143e+00
 * time: 226.112459897995
   200     1.068001e+03     3.885827e+00
 * time: 226.94122910499573
   201     1.067999e+03     5.022667e-01
 * time: 227.59934401512146
   202     1.068000e+03     6.120586e-01
 * time: 239.20092797279358
   203     1.067998e+03     6.225937e-01
 * time: 251.6999180316925
FittedPumasModel

Dynamical system type:               Nonlinear ODE
Solver(s): (OrdinaryDiffEqVerner.Vern7,OrdinaryDiffEqRosenbrock.Rodas5P)

Number of subjects:                             32

Observation records:         Active        Missing
    conc:                       251             47
    pca:                        232             66
    Total:                      483            113

Number of parameters:      Constant      Optimized
                                  0             18

Likelihood approximation:                     FOCE
Likelihood optimizer:                         BFGS

Termination Reason:                      NoXChange
Log-likelihood value:                   -1067.9978

-----------------------
            Estimate
-----------------------
pop_CL       0.13522
pop_V        8.0114
pop_tabs     0.57138
pop_lag      0.87464
pop_e0      96.409
pop_emax    -1.06
pop_c50      1.4826
pop_tover   14.008
pk_Ω₁,₁      0.067892
pk_Ω₂,₂      0.021036
pk_Ω₃,₃      0.84945
pd_Ω₁,₁      0.0029348
pd_Ω₂,₂      2.5044e-6
pd_Ω₃,₃      0.14671
pd_Ω₄,₄      0.015478
σ_prop       0.088415
σ_add        0.41724
σ_fx         3.581
-----------------------

but fares much better with lower tolerances of 1e-8 (relative) and 1e-12 (absolute):

fit(
    warfarin_pkpd_model,
    pop,
    init_params(warfarin_pkpd_model),
    FOCE();
    diffeq_options = (; reltol = 1e-8, abstol = 1e-12),
)
[ Info: Checking the initial parameter values.
[ Info: The initial negative log-likelihood and its gradient are finite. Check passed.
Iter     Function value   Gradient norm 
     0     3.125741e+06     5.911802e+06
 * time: 2.5033950805664062e-5
     1     5.174461e+05     8.708698e+05
 * time: 1.5654969215393066
     2     3.865265e+05     6.344302e+05
 * time: 2.9129581451416016
     3     1.804274e+05     2.829723e+05
 * time: 4.104030132293701
     4     9.706640e+04     1.550547e+05
 * time: 5.3237950801849365
     5     4.769637e+04     6.778817e+04
 * time: 6.516402959823608
     6     2.902319e+04     3.499747e+04
 * time: 7.609928131103516
     7     1.823472e+04     1.705751e+04
 * time: 8.730916976928711
     8     1.258819e+04     9.569381e+03
 * time: 9.939985036849976
     9     9.389984e+03     8.615851e+03
 * time: 11.066648006439209
    10     7.314702e+03     7.636883e+03
 * time: 12.179367065429688
    11     5.916029e+03     6.624325e+03
 * time: 13.29023003578186
    12     4.930519e+03     5.558140e+03
 * time: 14.400984048843384
    13     4.125060e+03     4.315759e+03
 * time: 15.518592119216919
    14     3.549280e+03     3.051093e+03
 * time: 16.719094038009644
    15     3.283489e+03     2.157292e+03
 * time: 17.804913997650146
    16     3.204886e+03     1.659798e+03
 * time: 18.85201406478882
    17     3.194875e+03     1.480528e+03
 * time: 19.96847891807556
    18     3.193944e+03     1.437921e+03
 * time: 21.102582931518555
    19     3.193070e+03     1.411186e+03
 * time: 22.187807083129883
    20     3.190129e+03     1.355327e+03
 * time: 23.34429097175598
    21     3.183228e+03     1.276603e+03
 * time: 24.608520030975342
    22     3.164897e+03     1.151838e+03
 * time: 25.74379801750183
    23     3.119250e+03     9.712651e+02
 * time: 26.949666023254395
    24     3.006297e+03     7.204342e+02
 * time: 28.059625148773193
    25     2.738913e+03     4.050545e+02
 * time: 29.104063034057617
    26     2.123834e+03     2.318194e+02
 * time: 30.008044004440308
    27     1.350274e+03     1.386111e+02
 * time: 31.375566005706787
    28     1.292769e+03     2.316810e+02
 * time: 32.86082100868225
    29     1.273651e+03     1.664584e+02
 * time: 33.74436593055725
    30     1.263845e+03     1.764773e+02
 * time: 34.633508920669556
    31     1.243264e+03     1.918458e+02
 * time: 35.485023975372314
    32     1.240721e+03     1.767778e+02
 * time: 36.34144401550293
    33     1.240528e+03     1.714649e+02
 * time: 37.18366312980652
    34     1.240516e+03     1.711394e+02
 * time: 38.01062202453613
    35     1.240469e+03     1.701233e+02
 * time: 38.876343965530396
    36     1.240367e+03     1.683191e+02
 * time: 39.74546813964844
    37     1.240086e+03     1.640882e+02
 * time: 40.62927198410034
    38     1.239406e+03     1.549688e+02
 * time: 41.578848123550415
    39     1.237810e+03     1.353098e+02
 * time: 42.48259496688843
    40     1.234651e+03     9.788573e+01
 * time: 43.38705611228943
    41     1.230250e+03     4.266570e+01
 * time: 44.23580002784729
    42     1.227165e+03     4.913304e+01
 * time: 45.08145308494568
    43     1.226203e+03     5.435433e+01
 * time: 45.91524910926819
    44     1.226090e+03     4.781720e+01
 * time: 46.74526596069336
    45     1.226088e+03     4.634324e+01
 * time: 47.59111404418945
    46     1.226087e+03     4.601513e+01
 * time: 48.48559093475342
    47     1.226081e+03     4.453279e+01
 * time: 49.32145094871521
    48     1.226069e+03     4.248078e+01
 * time: 50.185007095336914
    49     1.226037e+03     3.820894e+01
 * time: 51.03336501121521
    50     1.225957e+03     3.039316e+01
 * time: 51.90381908416748
    51     1.225769e+03     2.845915e+01
 * time: 52.74452209472656
    52     1.225398e+03     2.944618e+01
 * time: 53.57852792739868
    53     1.224884e+03     3.649371e+01
 * time: 54.43102502822876
    54     1.224523e+03     5.005295e+01
 * time: 55.277178049087524
    55     1.224424e+03     4.581771e+01
 * time: 56.18644714355469
    56     1.224413e+03     4.045804e+01
 * time: 57.069414138793945
    57     1.224411e+03     3.898760e+01
 * time: 57.899819135665894
    58     1.224409e+03     3.740817e+01
 * time: 58.73792099952698
    59     1.224403e+03     3.465864e+01
 * time: 59.59060096740723
    60     1.224387e+03     3.207203e+01
 * time: 60.44272494316101
    61     1.224344e+03     3.010607e+01
 * time: 61.33860206604004
    62     1.224234e+03     2.815612e+01
 * time: 62.187644958496094
    63     1.223949e+03     2.819773e+01
 * time: 63.06050205230713
    64     1.223237e+03     3.622961e+01
 * time: 63.90958094596863
    65     1.221600e+03     7.594055e+01
 * time: 64.75888705253601
    66     1.218599e+03     1.106912e+02
 * time: 65.6617419719696
    67     1.215109e+03     1.024965e+02
 * time: 66.52719807624817
    68     1.212822e+03     8.616050e+01
 * time: 67.39629292488098
    69     1.212285e+03     8.967803e+01
 * time: 68.28398609161377
    70     1.212264e+03     8.932132e+01
 * time: 69.12013006210327
    71     1.212260e+03     8.896522e+01
 * time: 69.93823003768921
    72     1.212244e+03     8.763366e+01
 * time: 70.78216600418091
    73     1.212211e+03     8.567001e+01
 * time: 71.64305806159973
    74     1.212117e+03     8.128684e+01
 * time: 72.47710013389587
    75     1.211887e+03     7.260216e+01
 * time: 73.34248805046082
    76     1.211324e+03     5.425869e+01
 * time: 74.20944595336914
    77     1.210115e+03     6.776024e+01
 * time: 75.08428597450256
    78     1.208108e+03     8.001108e+01
 * time: 75.93232011795044
    79     1.206180e+03     9.050256e+01
 * time: 76.79371905326843
    80     1.205414e+03     9.882265e+01
 * time: 77.66950798034668
    81     1.205304e+03     8.686897e+01
 * time: 78.5058810710907
    82     1.205297e+03     8.274517e+01
 * time: 79.3553159236908
    83     1.205293e+03     8.123277e+01
 * time: 80.25385904312134
    84     1.205277e+03     7.756801e+01
 * time: 81.13073396682739
    85     1.205243e+03     7.268799e+01
 * time: 81.9905469417572
    86     1.205146e+03     6.634996e+01
 * time: 82.84709310531616
    87     1.204904e+03     6.323779e+01
 * time: 83.68327307701111
    88     1.204289e+03     5.674357e+01
 * time: 84.55984306335449
    89     1.202870e+03     5.241928e+01
 * time: 85.51994514465332
    90     1.200212e+03     5.141237e+01
 * time: 86.36468696594238
    91     1.197000e+03     7.220230e+01
 * time: 87.3042299747467
    92     1.194663e+03     4.802449e+01
 * time: 88.24461197853088
    93     1.193885e+03     4.877522e+01
 * time: 89.12818098068237
    94     1.193833e+03     4.999065e+01
 * time: 89.92944312095642
    95     1.193831e+03     5.008260e+01
 * time: 90.7244040966034
    96     1.193830e+03     5.013799e+01
 * time: 91.51061010360718
    97     1.193824e+03     5.023264e+01
 * time: 92.30193710327148
    98     1.193812e+03     5.037089e+01
 * time: 93.1235089302063
    99     1.193778e+03     5.056811e+01
 * time: 94.02658104896545
   100     1.193690e+03     5.079476e+01
 * time: 94.90433192253113
   101     1.193465e+03     5.091049e+01
 * time: 95.75796914100647
   102     1.192908e+03     5.045173e+01
 * time: 96.64663410186768
   103     1.191643e+03     4.826733e+01
 * time: 97.51607203483582
   104     1.189287e+03     4.966444e+01
 * time: 98.47149705886841
   105     1.186354e+03     8.019652e+01
 * time: 99.41906809806824
   106     1.184499e+03     8.997361e+01
 * time: 100.25601291656494
   107     1.184032e+03     8.409142e+01
 * time: 101.09798407554626
   108     1.183980e+03     7.941840e+01
 * time: 101.92823100090027
   109     1.183972e+03     7.800909e+01
 * time: 102.81652903556824
   110     1.183969e+03     7.789073e+01
 * time: 103.6474039554596
   111     1.183965e+03     7.827288e+01
 * time: 104.47205209732056
   112     1.183957e+03     7.914384e+01
 * time: 105.33942008018494
   113     1.183941e+03     8.062266e+01
 * time: 106.26173400878906
   114     1.183902e+03     8.300645e+01
 * time: 107.15159201622009
   115     1.183800e+03     8.655837e+01
 * time: 108.02165603637695
   116     1.183541e+03     9.123920e+01
 * time: 108.88705611228943
   117     1.182906e+03     9.549885e+01
 * time: 109.7315719127655
   118     1.181503e+03     9.386248e+01
 * time: 110.57409310340881
   119     1.179134e+03     7.587261e+01
 * time: 111.6832001209259
   120     1.176807e+03     4.015019e+01
 * time: 112.56940412521362
   121     1.175726e+03     1.893891e+01
 * time: 113.48626708984375
   122     1.175462e+03     1.993836e+01
 * time: 114.40321803092957
   123     1.175429e+03     1.972240e+01
 * time: 115.3362820148468
   124     1.175427e+03     1.981999e+01
 * time: 116.25580310821533
   125     1.175426e+03     1.966086e+01
 * time: 117.1625280380249
   126     1.175426e+03     1.968277e+01
 * time: 118.0519289970398
   127     1.175424e+03     1.968319e+01
 * time: 118.94418907165527
   128     1.175421e+03     1.974292e+01
 * time: 119.79079699516296
   129     1.175412e+03     1.984730e+01
 * time: 120.65187406539917
   130     1.175390e+03     2.002879e+01
 * time: 121.51699209213257
   131     1.175331e+03     2.031908e+01
 * time: 122.40993809700012
   132     1.175176e+03     2.075996e+01
 * time: 123.28043508529663
   133     1.174766e+03     2.137371e+01
 * time: 124.17863512039185
   134     1.173682e+03     2.425284e+01
 * time: 125.13869595527649
   135     1.170870e+03     4.399728e+01
 * time: 126.10479998588562
   136     1.164080e+03     6.850329e+01
 * time: 127.13975405693054
   137     1.153404e+03     5.019046e+01
 * time: 128.2863941192627
   138     1.148838e+03     1.017073e+02
 * time: 129.42170095443726
   139     1.146757e+03     4.472171e+01
 * time: 130.733225107193
   140     1.145077e+03     2.419303e+01
 * time: 131.72443008422852
   141     1.142868e+03     4.737016e+01
 * time: 132.65195202827454
   142     1.142013e+03     3.798302e+01
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   143     1.141124e+03     2.039545e+01
 * time: 134.65999698638916
   144     1.140996e+03     2.088957e+01
 * time: 135.68250703811646
   145     1.140977e+03     2.088137e+01
 * time: 136.67269802093506
   146     1.140975e+03     2.100483e+01
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   147     1.140975e+03     2.099463e+01
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   148     1.140975e+03     2.098670e+01
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   149     1.140974e+03     2.098287e+01
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   150     1.140973e+03     2.097843e+01
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   151     1.140970e+03     2.097297e+01
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   152     1.140964e+03     2.096408e+01
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   153     1.140946e+03     2.094990e+01
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   154     1.140902e+03     2.092943e+01
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   155     1.140790e+03     2.090361e+01
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   156     1.140511e+03     2.083496e+01
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   157     1.139843e+03     2.015856e+01
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   158     1.138652e+03     1.563386e+01
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   159     1.136642e+03     1.940663e+01
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   160     1.130881e+03     3.438761e+01
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   161     1.127116e+03     2.732724e+01
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   162     1.121172e+03     2.329341e+01
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   163     1.114032e+03     3.693136e+01
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   164     1.113083e+03     1.625969e+01
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   165     1.111657e+03     9.839735e+00
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   166     1.111245e+03     1.672316e+01
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   167     1.111200e+03     1.011289e+01
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   168     1.111189e+03     1.126428e+01
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   169     1.111187e+03     1.072074e+01
 * time: 162.4282250404358
   170     1.111187e+03     1.058154e+01
 * time: 163.54794192314148
   171     1.111187e+03     1.059935e+01
 * time: 164.66437292099
   172     1.111186e+03     1.066755e+01
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   173     1.111185e+03     1.073422e+01
 * time: 166.95971202850342
   174     1.111182e+03     1.084102e+01
 * time: 168.12472796440125
   175     1.111174e+03     1.099326e+01
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   176     1.111153e+03     1.122224e+01
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   177     1.111099e+03     1.155538e+01
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   178     1.110959e+03     1.202824e+01
 * time: 172.85179805755615
   179     1.110607e+03     1.265924e+01
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   180     1.109748e+03     1.342521e+01
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   181     1.107812e+03     1.843518e+01
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   182     1.104084e+03     2.482762e+01
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   183     1.098538e+03     3.156038e+01
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   184     1.093041e+03     2.102741e+01
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   185     1.092139e+03     4.376057e+00
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   186     1.091949e+03     4.214735e+00
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   187     1.091924e+03     4.423897e+00
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   188     1.091915e+03     4.379812e+00
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   189     1.091911e+03     4.325356e+00
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   190     1.091907e+03     4.301718e+00
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   191     1.091905e+03     4.312479e+00
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   192     1.091904e+03     4.333231e+00
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   193     1.091904e+03     4.341267e+00
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   194     1.091904e+03     4.344148e+00
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   195     1.091904e+03     4.347377e+00
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   196     1.091904e+03     4.350432e+00
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   197     1.091904e+03     4.354607e+00
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   198     1.091903e+03     4.360357e+00
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   199     1.091902e+03     4.368853e+00
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   200     1.091899e+03     4.728219e+00
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   201     1.091891e+03     7.600915e+00
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   202     1.091869e+03     1.221674e+01
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   203     1.091814e+03     1.955276e+01
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   204     1.091671e+03     3.083649e+01
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   205     1.091313e+03     4.663244e+01
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   206     1.090503e+03     6.282148e+01
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   207     1.089183e+03     6.388542e+01
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   208     1.088151e+03     4.909379e+01
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   209     1.087509e+03     3.494681e+01
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   210     1.087080e+03     7.718616e+00
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   211     1.087045e+03     5.434022e+00
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   212     1.087043e+03     5.392509e+00
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   213     1.087043e+03     5.381737e+00
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   214     1.087043e+03     5.383544e+00
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   215     1.087043e+03     5.384736e+00
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   216     1.087043e+03     5.389168e+00
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   217     1.087043e+03     5.395864e+00
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   218     1.087042e+03     5.407975e+00
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   219     1.087041e+03     5.428555e+00
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   220     1.087039e+03     5.465713e+00
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   221     1.087033e+03     5.535143e+00
 * time: 230.94412302970886
   222     1.087016e+03     5.674084e+00
 * time: 232.2751989364624
   223     1.086972e+03     8.532540e+00
 * time: 233.55292391777039
   224     1.086848e+03     1.461753e+01
 * time: 234.83977603912354
   225     1.086412e+03     2.842371e+01
 * time: 236.07178807258606
   226     1.085222e+03     6.994075e+01
 * time: 238.6683099269867
   227     1.083851e+03     6.134735e+01
 * time: 239.84890294075012
   228     1.082442e+03     6.257404e+01
 * time: 241.45631408691406
   229     1.077492e+03     7.243971e+01
 * time: 243.0501389503479
   230     1.075473e+03     4.639961e+01
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   231     1.073275e+03     2.264633e+01
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   232     1.071725e+03     1.828391e+01
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   233     1.070530e+03     1.749938e+01
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   234     1.069856e+03     1.401865e+01
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   235     1.069530e+03     2.540004e+00
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   236     1.069495e+03     2.324898e+00
 * time: 251.29384994506836
   237     1.069488e+03     2.238028e+00
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   238     1.069488e+03     2.234884e+00
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   239     1.069488e+03     2.235545e+00
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   240     1.069488e+03     2.235598e+00
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   241     1.069488e+03     2.235952e+00
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   242     1.069488e+03     2.236398e+00
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   243     1.069488e+03     2.237191e+00
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   244     1.069488e+03     2.238247e+00
 * time: 260.61712098121643
   245     1.069487e+03     2.239436e+00
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   246     1.069486e+03     2.239775e+00
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   247     1.069484e+03     2.236150e+00
 * time: 264.31440711021423
   248     1.069478e+03     2.819848e+00
 * time: 265.4916410446167
   249     1.069462e+03     4.569597e+00
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   250     1.069423e+03     7.151931e+00
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   251     1.069337e+03     1.033894e+01
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   252     1.069182e+03     1.222813e+01
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   253     1.069012e+03     8.750861e+00
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   254     1.068940e+03     2.347851e+00
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   255     1.068930e+03     8.660232e-01
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   256     1.068929e+03     8.684927e-01
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   257     1.068929e+03     8.688773e-01
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   258     1.068929e+03     8.689364e-01
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   259     1.068929e+03     8.689906e-01
 * time: 278.7022521495819
   260     1.068929e+03     8.690639e-01
 * time: 279.82327008247375
   261     1.068929e+03     8.691862e-01
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   262     1.068929e+03     8.693731e-01
 * time: 282.1010820865631
   263     1.068929e+03     8.696634e-01
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   264     1.068929e+03     8.700941e-01
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   265     1.068929e+03     8.706948e-01
 * time: 285.52480006217957
   266     1.068929e+03     8.714109e-01
 * time: 286.66775012016296
   267     1.068928e+03     1.135042e+00
 * time: 287.85991191864014
   268     1.068925e+03     1.834310e+00
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   269     1.068918e+03     2.941732e+00
 * time: 290.7323660850525
   270     1.068901e+03     4.631750e+00
 * time: 292.5980670452118
   271     1.068860e+03     6.959147e+00
 * time: 294.00758695602417
   272     1.068771e+03     9.324762e+00
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   273     1.068620e+03     9.719454e+00
 * time: 296.4024999141693
   274     1.068450e+03     6.253902e+00
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   275     1.068381e+03     2.060774e+00
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   276     1.068375e+03     1.329067e+00
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   277     1.068375e+03     1.303384e+00
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   278     1.068374e+03     1.285284e+00
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   279     1.068374e+03     1.283587e+00
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   280     1.068374e+03     1.283262e+00
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   281     1.068374e+03     1.282394e+00
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   282     1.068374e+03     1.281210e+00
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   283     1.068374e+03     1.279149e+00
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   284     1.068374e+03     1.275842e+00
 * time: 309.3928129673004
   285     1.068374e+03     1.270320e+00
 * time: 310.68015003204346
   286     1.068374e+03     1.261096e+00
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   287     1.068374e+03     1.245320e+00
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   288     1.068372e+03     1.217687e+00
 * time: 314.23217391967773
   289     1.068369e+03     1.880347e+00
 * time: 315.3807189464569
   290     1.068360e+03     2.989937e+00
 * time: 316.5441589355469
   291     1.068339e+03     4.564004e+00
 * time: 317.70784997940063
   292     1.068294e+03     6.255183e+00
 * time: 318.9276020526886
   293     1.068218e+03     6.693297e+00
 * time: 320.1140160560608
   294     1.068129e+03     4.691438e+00
 * time: 321.27065801620483
   295     1.068058e+03     1.778432e+00
 * time: 322.48358702659607
   296     1.068021e+03     1.060128e-01
 * time: 323.6565909385681
   297     1.068007e+03     5.380572e-01
 * time: 324.8908619880676
   298     1.068000e+03     4.880990e-01
 * time: 326.1059651374817
   299     1.067995e+03     2.128989e-01
 * time: 327.32036995887756
   300     1.067993e+03     5.531827e-02
 * time: 328.57333302497864
   301     1.067992e+03     8.171941e-02
 * time: 329.7879719734192
   302     1.067992e+03     7.780559e-02
 * time: 331.01171112060547
   303     1.067991e+03     8.837140e-02
 * time: 332.4144170284271
   304     1.067991e+03     9.376056e-02
 * time: 333.7503011226654
   305     1.067991e+03     9.513439e-02
 * time: 334.92954993247986
   306     1.067991e+03     9.526533e-02
 * time: 336.09844493865967
   307     1.067991e+03     9.529304e-02
 * time: 337.2295660972595
   308     1.067991e+03     9.528758e-02
 * time: 338.3707821369171
   309     1.067991e+03     9.527374e-02
 * time: 339.55013704299927
   310     1.067991e+03     9.524600e-02
 * time: 340.70316910743713
   311     1.067991e+03     9.519336e-02
 * time: 341.9980230331421
   312     1.067991e+03     9.511341e-02
 * time: 343.1991379261017
   313     1.067991e+03     9.495795e-02
 * time: 344.45249009132385
   314     1.067991e+03     9.467216e-02
 * time: 345.5718550682068
   315     1.067991e+03     9.410027e-02
 * time: 346.6997411251068
   316     1.067991e+03     9.294065e-02
 * time: 347.8513159751892
   317     1.067991e+03     1.029917e-01
 * time: 348.9983811378479
   318     1.067991e+03     1.610004e-01
 * time: 350.15007996559143
   319     1.067991e+03     2.392391e-01
 * time: 351.36393213272095
   320     1.067991e+03     3.102433e-01
 * time: 352.6082510948181
   321     1.067991e+03     3.039617e-01
 * time: 353.76532006263733
   322     1.067990e+03     1.709451e-01
 * time: 354.9542829990387
   323     1.067990e+03     3.340140e-02
 * time: 356.10167598724365
   324     1.067990e+03     1.340511e-02
 * time: 357.2319369316101
   325     1.067990e+03     2.463356e-02
 * time: 358.380912065506
   326     1.067990e+03     2.689690e-02
 * time: 359.5507061481476
   327     1.067990e+03     1.754625e-02
 * time: 360.7076749801636
   328     1.067990e+03     4.559747e-03
 * time: 361.8265149593353
   329     1.067990e+03     2.117925e-03
 * time: 362.96642994880676
   330     1.067990e+03     3.040111e-03
 * time: 364.09634494781494
   331     1.067990e+03     2.450615e-03
 * time: 365.1975030899048
   332     1.067990e+03     6.637513e-04
 * time: 366.32954692840576
FittedPumasModel

Dynamical system type:               Nonlinear ODE
Solver(s): (OrdinaryDiffEqVerner.Vern7,OrdinaryDiffEqRosenbrock.Rodas5P)

Number of subjects:                             32

Observation records:         Active        Missing
    conc:                       251             47
    pca:                        232             66
    Total:                      483            113

Number of parameters:      Constant      Optimized
                                  0             18

Likelihood approximation:                     FOCE
Likelihood optimizer:                         BFGS

Termination Reason:                   GradientNorm
Log-likelihood value:                   -1067.9901

------------------------
            Estimate
------------------------
pop_CL       0.13521
pop_V        8.0131
pop_tabs     0.57176
pop_lag      0.87544
pop_e0      96.401
pop_emax    -1.0614
pop_c50      1.4902
pop_tover   14.047
pk_Ω₁,₁      0.067971
pk_Ω₂,₂      0.021022
pk_Ω₃,₃      0.85621
pd_Ω₁,₁      0.0029367
pd_Ω₂,₂      1.9257e-10
pd_Ω₃,₃      0.14607
pd_Ω₄,₄      0.01538
σ_prop       0.088485
σ_add        0.41684
σ_fx         3.5814
------------------------
Tip

It is not recommended to decrease tolerances below 1e-14.

7.1 Changing the Algorithm

Usually, it should not be necessary to adjust the differential equation solver. If you change the solver, you should follow the guidelines in the SciML documentation that explains which solvers are the most efficient at the desired tolerance level.

For instance, if it is known that a differential equation is stiff, a stiff solver such as Rosenbrock23 at high tolerances or Rodas5P at low tolerances could be a possible alternative to the default auto-switching solver:

# Fitting with stiff solver Rodas5P at low tolerances (relative: 1e-8, absolute: 1e-12)
fit(
    warfarin_pkpd_model,
    pop,
    init_params(warfarin_pkpd_model),
    FOCE();
    diffeq_options = (; alg = Rodas5P(), reltol = 1e-8, abstol = 1e-12),
)
[ Info: Checking the initial parameter values.
[ Info: The initial negative log-likelihood and its gradient are finite. Check passed.
Iter     Function value   Gradient norm 
     0     3.125741e+06     5.911803e+06
 * time: 0.00016999244689941406
     1     5.174461e+05     8.708699e+05
 * time: 14.550684928894043
     2     3.865265e+05     6.344302e+05
 * time: 20.07855486869812
     3     1.804274e+05     2.829723e+05
 * time: 25.854876041412354
     4     9.706641e+04     1.550547e+05
 * time: 30.83341383934021
     5     4.769637e+04     6.778818e+04
 * time: 38.68268084526062
     6     2.902319e+04     3.499747e+04
 * time: 43.69065594673157
     7     1.823472e+04     1.705751e+04
 * time: 47.99008393287659
     8     1.258819e+04     9.569382e+03
 * time: 52.87374496459961
     9     9.389985e+03     8.615853e+03
 * time: 57.113117933273315
    10     7.314703e+03     7.636886e+03
 * time: 61.480814933776855
    11     5.916030e+03     6.624328e+03
 * time: 66.43713188171387
    12     4.930520e+03     5.558143e+03
 * time: 70.61531090736389
    13     4.125062e+03     4.315761e+03
 * time: 74.82996487617493
    14     3.549280e+03     3.051094e+03
 * time: 79.23385405540466
    15     3.283490e+03     2.157293e+03
 * time: 83.44006395339966
    16     3.204886e+03     1.659798e+03
 * time: 87.89565205574036
    17     3.194875e+03     1.480528e+03
 * time: 92.04142189025879
    18     3.193944e+03     1.437922e+03
 * time: 96.44697093963623
    19     3.193070e+03     1.411186e+03
 * time: 100.71159887313843
    20     3.190129e+03     1.355327e+03
 * time: 105.06885695457458
    21     3.183228e+03     1.276603e+03
 * time: 109.23470187187195
    22     3.164897e+03     1.151838e+03
 * time: 113.48449397087097
    23     3.119250e+03     9.712652e+02
 * time: 117.74986791610718
    24     3.006297e+03     7.204344e+02
 * time: 122.58709406852722
    25     2.738913e+03     4.050546e+02
 * time: 126.72298693656921
    26     2.123834e+03     2.318194e+02
 * time: 130.802307844162
    27     1.350274e+03     1.386114e+02
 * time: 137.1480839252472
    28     1.292769e+03     2.316788e+02
 * time: 143.45056796073914
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 * time: 1159.851233959198
   267     1.068927e+03     2.041852e+00
 * time: 1164.214642047882
   268     1.068924e+03     3.317723e+00
 * time: 1169.7155039310455
   269     1.068916e+03     5.340187e+00
 * time: 1174.0935220718384
   270     1.068895e+03     8.406870e+00
 * time: 1178.0676238536835
   271     1.068846e+03     1.254941e+01
 * time: 1182.024672985077
   272     1.068743e+03     1.646883e+01
 * time: 1186.0180139541626
   273     1.068575e+03     1.626631e+01
 * time: 1189.9859020709991
   274     1.068421e+03     9.263060e+00
 * time: 1194.0145299434662
   275     1.068378e+03     2.722057e+00
 * time: 1198.052186012268
   276     1.068375e+03     1.322919e+00
 * time: 1201.981628894806
   277     1.068375e+03     1.298790e+00
 * time: 1206.6636128425598
   278     1.068374e+03     1.285062e+00
 * time: 1212.9024710655212
   279     1.068374e+03     1.283538e+00
 * time: 1216.9659378528595
   280     1.068374e+03     1.283199e+00
 * time: 1220.8219559192657
   281     1.068374e+03     1.282304e+00
 * time: 1224.990040063858
   282     1.068374e+03     1.281085e+00
 * time: 1228.880192041397
   283     1.068374e+03     1.278801e+00
 * time: 1232.980311870575
   284     1.068374e+03     1.275266e+00
 * time: 1237.0499558448792
   285     1.068374e+03     1.269275e+00
 * time: 1241.0802400112152
   286     1.068374e+03     1.259302e+00
 * time: 1244.9398288726807
   287     1.068374e+03     1.242172e+00
 * time: 1248.7994899749756
   288     1.068372e+03     1.401112e+00
 * time: 1252.700157880783
   289     1.068368e+03     2.260428e+00
 * time: 1256.577101945877
   290     1.068359e+03     3.584961e+00
 * time: 1260.4509680271149
   291     1.068336e+03     5.440145e+00
 * time: 1264.606430053711
   292     1.068286e+03     7.356942e+00
 * time: 1268.5921039581299
   293     1.068206e+03     7.641557e+00
 * time: 1272.6450338363647
   294     1.068116e+03     4.984124e+00
 * time: 1276.5727789402008
   295     1.068049e+03     1.625105e+00
 * time: 1280.466562986374
   296     1.068017e+03     2.028511e-01
 * time: 1284.4449570178986
   297     1.068006e+03     6.751563e-01
 * time: 1288.3944828510284
   298     1.067999e+03     5.907583e-01
 * time: 1292.259516954422
   299     1.067995e+03     2.558128e-01
 * time: 1296.1434450149536
   300     1.067993e+03     5.751238e-02
 * time: 1300.0141320228577
   301     1.067992e+03     1.010087e-01
 * time: 1303.7901890277863
   302     1.067992e+03     8.610794e-02
 * time: 1307.4932289123535
   303     1.067991e+03     8.997398e-02
 * time: 1311.2728679180145
   304     1.067991e+03     9.405667e-02
 * time: 1315.0712490081787
   305     1.067991e+03     9.506296e-02
 * time: 1318.832669019699
   306     1.067991e+03     9.519565e-02
 * time: 1322.6582880020142
   307     1.067991e+03     9.524886e-02
 * time: 1326.5364608764648
   308     1.067991e+03     9.527318e-02
 * time: 1330.519276857376
   309     1.067991e+03     9.530136e-02
 * time: 1334.303118944168
   310     1.067991e+03     9.534196e-02
 * time: 1338.1347830295563
   311     1.067991e+03     9.538292e-02
 * time: 1341.955505847931
   312     1.067991e+03     9.549037e-02
 * time: 1345.7550508975983
   313     1.067991e+03     9.559000e-02
 * time: 1350.1497700214386
   314     1.067991e+03     9.575797e-02
 * time: 1354.0059170722961
   315     1.067991e+03     9.588288e-02
 * time: 1357.8614840507507
   316     1.067991e+03     9.577851e-02
 * time: 1361.7445268630981
   317     1.067991e+03     1.228736e-01
 * time: 1365.622586965561
   318     1.067991e+03     1.927365e-01
 * time: 1369.7620329856873
   319     1.067991e+03     2.828484e-01
 * time: 1373.9364738464355
   320     1.067991e+03     3.568974e-01
 * time: 1378.1144468784332
   321     1.067990e+03     3.280426e-01
 * time: 1384.0172319412231
   322     1.067990e+03     1.629326e-01
 * time: 1387.9558148384094
   323     1.067990e+03     2.262745e-02
 * time: 1392.605318069458
   324     1.067990e+03     1.806059e-02
 * time: 1396.585224866867
   325     1.067990e+03     2.892375e-02
 * time: 1400.499295949936
   326     1.067990e+03     2.939738e-02
 * time: 1404.3604400157928
   327     1.067990e+03     1.819943e-02
 * time: 1408.2225549221039
   328     1.067990e+03     3.826783e-03
 * time: 1412.0905890464783
   329     1.067990e+03     2.380260e-03
 * time: 1415.9229929447174
   330     1.067990e+03     3.364719e-03
 * time: 1419.940201997757
   331     1.067990e+03     2.287408e-03
 * time: 1424.975431919098
   332     1.067990e+03     7.907173e-04
 * time: 1430.074373960495
FittedPumasModel

Dynamical system type:               Nonlinear ODE
Solver(s):        OrdinaryDiffEqRosenbrock.Rodas5P

Number of subjects:                             32

Observation records:         Active        Missing
    conc:                       251             47
    pca:                        232             66
    Total:                      483            113

Number of parameters:      Constant      Optimized
                                  0             18

Likelihood approximation:                     FOCE
Likelihood optimizer:                         BFGS

Termination Reason:                   GradientNorm
Log-likelihood value:                   -1067.9901

------------------------
            Estimate
------------------------
pop_CL       0.13521
pop_V        8.0131
pop_tabs     0.57176
pop_lag      0.87544
pop_e0      96.401
pop_emax    -1.0614
pop_c50      1.4903
pop_tover   14.047
pk_Ω₁,₁      0.067971
pk_Ω₂,₂      0.021022
pk_Ω₃,₃      0.85621
pd_Ω₁,₁      0.0029368
pd_Ω₂,₂      1.7943e-10
pd_Ω₃,₃      0.14607
pd_Ω₄,₄      0.01538
σ_prop       0.088485
σ_add        0.41684
σ_fx         3.5814
------------------------

On the other hand, if it is known that a differential equation is non-stiff (this might be difficult to guarantee for all admissible parameter values), a non-stiff solver such as Tsit5 at high tolerances or Vern7 at low tolerances could be an alternative to the default solver:

# Fitting with the non-stiff solver Vern7 at low tolerances (relative: 1e-8, absolute: 1e-12)
fit(
    warfarin_pkpd_model,
    pop,
    init_params(warfarin_pkpd_model),
    FOCE();
    diffeq_options = (; alg = Vern7(), reltol = 1e-8, abstol = 1e-12),
)
[ Info: Checking the initial parameter values.
[ Info: The initial negative log-likelihood and its gradient are finite. Check passed.
Iter     Function value   Gradient norm 
     0     3.125741e+06     5.911802e+06
 * time: 2.5033950805664062e-5
     1     5.174461e+05     8.708698e+05
 * time: 5.074887990951538
     2     3.865265e+05     6.344302e+05
 * time: 6.147215127944946
     3     1.804274e+05     2.829723e+05
 * time: 7.145032167434692
     4     9.706640e+04     1.550547e+05
 * time: 8.020995140075684
     5     4.769637e+04     6.778817e+04
 * time: 8.883856058120728
     6     2.902319e+04     3.499747e+04
 * time: 9.730170011520386
     7     1.823472e+04     1.705751e+04
 * time: 10.857994079589844
     8     1.258819e+04     9.569381e+03
 * time: 11.885348081588745
     9     9.389984e+03     8.615851e+03
 * time: 12.760809183120728
    10     7.314702e+03     7.636883e+03
 * time: 13.60620903968811
    11     5.916029e+03     6.624325e+03
 * time: 14.44742202758789
    12     4.930519e+03     5.558140e+03
 * time: 15.27165699005127
    13     4.125060e+03     4.315759e+03
 * time: 16.078763961791992
    14     3.549280e+03     3.051093e+03
 * time: 16.91045117378235
    15     3.283489e+03     2.157292e+03
 * time: 17.71856713294983
    16     3.204886e+03     1.659798e+03
 * time: 18.523853063583374
    17     3.194875e+03     1.480528e+03
 * time: 19.3107750415802
    18     3.193944e+03     1.437921e+03
 * time: 20.120126008987427
    19     3.193070e+03     1.411186e+03
 * time: 20.91841197013855
    20     3.190129e+03     1.355327e+03
 * time: 21.703907012939453
    21     3.183228e+03     1.276603e+03
 * time: 22.504170179367065
    22     3.164897e+03     1.151838e+03
 * time: 23.31883716583252
    23     3.119250e+03     9.712651e+02
 * time: 24.155182123184204
    24     3.006297e+03     7.204342e+02
 * time: 24.94045114517212
    25     2.738913e+03     4.050545e+02
 * time: 25.705734968185425
    26     2.123834e+03     2.318194e+02
 * time: 26.4302020072937
    27     1.350274e+03     1.386112e+02
 * time: 27.56884002685547
    28     1.292769e+03     2.316796e+02
 * time: 28.720519065856934
    29     1.273651e+03     1.664572e+02
 * time: 29.43631601333618
    30     1.263845e+03     1.764763e+02
 * time: 30.162296056747437
    31     1.243263e+03     1.918451e+02
 * time: 30.889852046966553
    32     1.240721e+03     1.767770e+02
 * time: 31.606821060180664
    33     1.240528e+03     1.714639e+02
 * time: 32.33573913574219
    34     1.240516e+03     1.711384e+02
 * time: 33.035346031188965
    35     1.240469e+03     1.701225e+02
 * time: 33.765491008758545
    36     1.240367e+03     1.683186e+02
 * time: 34.48118710517883
    37     1.240086e+03     1.640884e+02
 * time: 35.18774700164795
    38     1.239406e+03     1.549705e+02
 * time: 35.90207815170288
    39     1.237810e+03     1.353142e+02
 * time: 36.638734102249146
    40     1.234651e+03     9.789409e+01
 * time: 37.40723896026611
    41     1.230250e+03     4.267605e+01
 * time: 38.14742398262024
    42     1.227165e+03     4.912917e+01
 * time: 38.86614108085632
    43     1.226203e+03     5.435504e+01
 * time: 39.57666611671448
    44     1.226090e+03     4.781765e+01
 * time: 40.316648960113525
    45     1.226088e+03     4.634307e+01
 * time: 41.01972198486328
    46     1.226087e+03     4.601501e+01
 * time: 41.71532607078552
    47     1.226081e+03     4.453260e+01
 * time: 42.42231202125549
    48     1.226069e+03     4.248058e+01
 * time: 43.121623039245605
    49     1.226037e+03     3.820863e+01
 * time: 43.85700011253357
    50     1.225957e+03     3.039268e+01
 * time: 44.57602095603943
    51     1.225769e+03     2.845915e+01
 * time: 45.3095760345459
    52     1.225398e+03     2.944634e+01
 * time: 46.015511989593506
    53     1.224884e+03     3.649468e+01
 * time: 46.724472999572754
    54     1.224523e+03     5.005317e+01
 * time: 47.46618700027466
    55     1.224424e+03     4.581761e+01
 * time: 48.17306900024414
    56     1.224413e+03     4.045809e+01
 * time: 48.88673114776611
    57     1.224411e+03     3.898771e+01
 * time: 49.5840220451355
    58     1.224409e+03     3.740820e+01
 * time: 50.284790992736816
    59     1.224403e+03     3.465864e+01
 * time: 51.01718497276306
    60     1.224387e+03     3.207196e+01
 * time: 51.72339701652527
    61     1.224344e+03     3.010594e+01
 * time: 52.42959713935852
    62     1.224234e+03     2.815612e+01
 * time: 53.133602142333984
    63     1.223949e+03     2.819773e+01
 * time: 53.83869910240173
    64     1.223237e+03     3.623110e+01
 * time: 54.57143807411194
    65     1.221600e+03     7.594252e+01
 * time: 55.29539513587952
    66     1.218599e+03     1.106926e+02
 * time: 56.004554986953735
    67     1.215108e+03     1.024959e+02
 * time: 56.70123815536499
    68     1.212822e+03     8.616184e+01
 * time: 57.42044711112976
    69     1.212285e+03     8.967904e+01
 * time: 58.11575508117676
    70     1.212264e+03     8.932231e+01
 * time: 58.80633211135864
    71     1.212260e+03     8.896622e+01
 * time: 59.50137901306152
    72     1.212243e+03     8.763463e+01
 * time: 60.189088106155396
    73     1.212211e+03     8.567093e+01
 * time: 60.90437197685242
    74     1.212117e+03     8.128762e+01
 * time: 61.62605118751526
    75     1.211887e+03     7.260263e+01
 * time: 62.318788051605225
    76     1.211323e+03     5.425847e+01
 * time: 63.00772404670715
    77     1.210115e+03     6.776239e+01
 * time: 63.71281599998474
    78     1.208108e+03     8.001211e+01
 * time: 64.4258291721344
    79     1.206179e+03     9.050308e+01
 * time: 65.11869215965271
    80     1.205413e+03     9.882075e+01
 * time: 65.81401300430298
    81     1.205304e+03     8.686728e+01
 * time: 66.49158096313477
    82     1.205297e+03     8.274394e+01
 * time: 67.18838500976562
    83     1.205293e+03     8.123149e+01
 * time: 67.91744804382324
    84     1.205277e+03     7.756682e+01
 * time: 68.61448812484741
    85     1.205243e+03     7.268681e+01
 * time: 69.3054780960083
    86     1.205146e+03     6.634955e+01
 * time: 69.9879560470581
    87     1.204904e+03     6.323739e+01
 * time: 70.6698579788208
    88     1.204289e+03     5.674317e+01
 * time: 71.37492609024048
    89     1.202870e+03     5.241823e+01
 * time: 72.06206202507019
    90     1.200212e+03     5.141235e+01
 * time: 72.74678015708923
    91     1.197000e+03     7.220159e+01
 * time: 73.43803000450134
    92     1.194663e+03     4.802355e+01
 * time: 74.13504815101624
    93     1.193885e+03     4.877511e+01
 * time: 74.83901810646057
    94     1.193833e+03     4.999053e+01
 * time: 75.52200508117676
    95     1.193831e+03     5.008249e+01
 * time: 76.1981680393219
    96     1.193830e+03     5.013788e+01
 * time: 76.8628900051117
    97     1.193824e+03     5.023254e+01
 * time: 77.54531002044678
    98     1.193812e+03     5.037081e+01
 * time: 78.23246097564697
    99     1.193778e+03     5.056804e+01
 * time: 78.9149911403656
   100     1.193690e+03     5.079472e+01
 * time: 79.58702802658081
   101     1.193465e+03     5.091051e+01
 * time: 80.26062417030334
   102     1.192908e+03     5.045181e+01
 * time: 80.97579908370972
   103     1.191643e+03     4.826748e+01
 * time: 81.67998504638672
   104     1.189287e+03     4.966410e+01
 * time: 82.4994101524353
   105     1.186354e+03     8.019602e+01
 * time: 83.4256420135498
   106     1.184499e+03     8.997306e+01
 * time: 84.36496114730835
   107     1.184032e+03     8.409107e+01
 * time: 85.41715812683105
   108     1.183980e+03     7.941815e+01
 * time: 86.40492916107178
   109     1.183972e+03     7.800888e+01
 * time: 87.33815908432007
   110     1.183969e+03     7.789060e+01
 * time: 88.18999600410461
   111     1.183965e+03     7.827285e+01
 * time: 88.99979496002197
   112     1.183957e+03     7.914388e+01
 * time: 89.83023810386658
   113     1.183941e+03     8.062278e+01
 * time: 90.6430561542511
   114     1.183901e+03     8.300665e+01
 * time: 91.48837304115295
   115     1.183800e+03     8.655866e+01
 * time: 92.29888010025024
   116     1.183541e+03     9.123963e+01
 * time: 93.13248205184937
   117     1.182906e+03     9.549957e+01
 * time: 93.95252704620361
   118     1.181504e+03     9.386385e+01
 * time: 94.77458596229553
   119     1.179134e+03     7.587507e+01
 * time: 95.49980807304382
   120     1.176807e+03     4.015289e+01
 * time: 96.20320105552673
   121     1.175726e+03     1.893886e+01
 * time: 96.92298698425293
   122     1.175462e+03     1.993822e+01
 * time: 97.64284706115723
   123     1.175429e+03     1.972233e+01
 * time: 98.52489399909973
   124     1.175427e+03     1.982000e+01
 * time: 99.51714205741882
   125     1.175426e+03     1.966081e+01
 * time: 100.49192500114441
   126     1.175426e+03     1.968272e+01
 * time: 101.5032811164856
   127     1.175424e+03     1.968319e+01
 * time: 102.41827416419983
   128     1.175421e+03     1.974296e+01
 * time: 103.36392498016357
   129     1.175412e+03     1.984742e+01
 * time: 104.22106313705444
   130     1.175390e+03     2.002902e+01
 * time: 105.05401396751404
   131     1.175331e+03     2.031948e+01
 * time: 105.90818309783936
   132     1.175176e+03     2.076063e+01
 * time: 106.76003909111023
   133     1.174766e+03     2.137481e+01
 * time: 107.60595297813416
   134     1.173683e+03     2.425753e+01
 * time: 108.4549331665039
   135     1.170871e+03     4.400799e+01
 * time: 109.38272500038147
   136     1.164081e+03     6.851982e+01
 * time: 110.27794313430786
   137     1.153405e+03     5.020585e+01
 * time: 111.04883408546448
   138     1.148838e+03     1.016698e+02
 * time: 111.89185309410095
   139     1.146758e+03     4.473168e+01
 * time: 112.99575901031494
   140     1.145077e+03     2.419327e+01
 * time: 113.76883602142334
   141     1.142868e+03     4.735929e+01
 * time: 114.52840518951416
   142     1.142013e+03     3.798978e+01
 * time: 115.30032110214233
   143     1.141124e+03     2.039426e+01
 * time: 116.05578207969666
   144     1.140996e+03     2.088977e+01
 * time: 116.80515503883362
   145     1.140977e+03     2.088121e+01
 * time: 117.54671502113342
   146     1.140975e+03     2.100463e+01
 * time: 118.28562712669373
   147     1.140975e+03     2.099458e+01
 * time: 119.03301405906677
   148     1.140975e+03     2.098676e+01
 * time: 119.76730298995972
   149     1.140974e+03     2.098300e+01
 * time: 120.5019371509552
   150     1.140973e+03     2.097870e+01
 * time: 121.25429511070251
   151     1.140970e+03     2.097346e+01
 * time: 122.02808904647827
   152     1.140964e+03     2.096496e+01
 * time: 122.76924109458923
   153     1.140946e+03     2.095149e+01
 * time: 123.5110981464386
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   325     1.067990e+03     8.765056e-04
 * time: 293.956915140152
FittedPumasModel

Dynamical system type:               Nonlinear ODE
Solver(s):              OrdinaryDiffEqVerner.Vern7

Number of subjects:                             32

Observation records:         Active        Missing
    conc:                       251             47
    pca:                        232             66
    Total:                      483            113

Number of parameters:      Constant      Optimized
                                  0             18

Likelihood approximation:                     FOCE
Likelihood optimizer:                         BFGS

Termination Reason:                   GradientNorm
Log-likelihood value:                   -1067.9901

-----------------------
            Estimate
-----------------------
pop_CL       0.13521
pop_V        8.0131
pop_tabs     0.57175
pop_lag      0.87544
pop_e0      96.401
pop_emax    -1.0614
pop_c50      1.4903
pop_tover   14.047
pk_Ω₁,₁      0.067972
pk_Ω₂,₂      0.021022
pk_Ω₃,₃      0.8563
pd_Ω₁,₁      0.0029372
pd_Ω₂,₂      1.4671e-9
pd_Ω₃,₃      0.14607
pd_Ω₄,₄      0.01538
σ_prop       0.088485
σ_add        0.41684
σ_fx         3.5814
-----------------------

8 Concluding Remarks

In this tutorial, you have seen how to adjust the tolerances and the algorithm of the differential solver. Usually, the default differential equation solver in Pumas is an efficient choice. To reduce numerical issues, sometimes it can be helpful to decrease the default tolerances.