# KL-Optimality: designs for model discrimination

    library(optedr)

## Background

Standard optimality criteria (D, A, I, …) assume the model is correctly specified and aim to estimate its parameters as precisely as possible. KL-Optimality addresses a different question: given two competing models, where should observations be taken to tell them apart most easily?

The criterion is based on the Kullback-Leibler divergence between the reference model \(f_1\) and the best-fitting version of the rival \(f_2\):

\[\Phi_{\text{KL}}(\xi) = \int \min_{\beta_2} \text{KL}(f_1(y \mid x) \,\|\, f_2(y \mid x, \beta_2))\, \xi(dx).\]

The design \(\xi^*\) that maximises \(\Phi_{\text{KL}}\) places observations where the two models are most distinguishable on average.

Reference: López-Fidalgo, Tommasi & Trandafir (2007). An optimal experimental design criterion for discriminating between non-normal models. J. R. Stat. Soc. B, 69(2), 231–242.

## API overview

    opt_des(
      "KL-Optimality",
      model        = <reference model formula>,
      parameters   = <reference parameter names>,
      par_values   = <nominal values>,
      design_space = <scalar interval or named list>,
      # rival specification:
      rival_model  = <rival formula>,       # default: same as model
      rival_params = <rival param names>,   # default: same as parameters
      rival_pars   = <starting values for internal optimisation>,
      rival_lower  = <lower bounds for rival params>,
      rival_upper  = <upper bounds for rival params>,
      # GLM family:
      family       = "Normal",   # "Poisson", "Binomial", "Gamma"
      phi          = 1,          # dispersion parameter
      # alternative: supply a custom KL function
      kl_fun       = NULL
    )

The internal optimisation over rival parameters is done automatically. The optimal rival values are stored as a hidden attribute and shown by `summary()`.

## Example 1: Michaelis-Menten vs linear rival (Normal, 1D)

Context. Enzyme kinetics with one substrate. The reference model is Michaelis-Menten \(\mu(x) = V_{\max} x / (K_m + x)\) with \(V_{\max} = 2\), \(K_m = 1\). The rival is a linear rate \(\mu(x) = a x\), valid only when \(x \ll K_m\). At high concentrations MM saturates while the linear model keeps growing — that is where the models diverge most.

    result_kl_mm <- opt_des(
      "KL-Optimality",
      model        = y ~ Vmax * x / (Km + x),
      parameters   = c("Vmax", "Km"),
      par_values   = c(2, 1),
      design_space = c(0.1, 5),
      rival_model  = y ~ a * x,
      rival_params = c("a"),
      rival_pars   = c(1),
      rival_lower  = c(0.2),
      rival_upper  = c(2.5),
      family       = "Normal",
      phi          = 1
    )
    #>
    #> ℹ Stop condition reached: difference between sensitivity and criterion < 1e-05
    #> ⠙ Calculating optimal design 7 done (5/s) | 1.4sℹ The lower bound for efficiency is 99.9957589718079%
    #>
    result_kl_mm
    #> Optimal design for KL-Optimality:
    #>      Point    Weight
    #> 1 1.127578 0.8159831
    #> 2 5.000000 0.1840169

`summary()` shows the GLM family and the optimal rival parameters found by the internal optimisation:

    summary(result_kl_mm)
    #> Model:
    #> y ~ Vmax * x/(Km + x)
    #> Weight function:
    #> function (x)
    #> 1
    #>
    #> GLM family: Normal  (phi = 1)
    #> Optimal rival parameters: 0.445
    #>
    #> Optimal design for KL-Optimality:
    #>      Point    Weight
    #> 1 1.127578 0.8159831
    #> 2 5.000000 0.1840169
    #>
    #> Minimum efficiency (Atwood): 99.9957589718079%
    #> Criterion value:             0.155804

The sensitivity plot shows \(d_{\text{KL}}(x, \xi)\) with the Equivalence Theorem bound (dashed). Support points concentrate in the saturation region where MM and the linear rival differ most:

    plot(result_kl_mm)

KL sensitivity function: MM vs linear rival.

Efficiency of a five-point uniform design relative to the KL-optimal:

    design_unif <- data.frame(
      Point  = c(0.1, 1.3, 2.5, 3.8, 5.0),
      Weight = rep(1/5, 5)
    )
    eff_kl <- design_efficiency(design_unif, result_kl_mm)
    #> ℹ The efficiency of the design is 44.1071528869909%
    cat("Efficiency of uniform design:", round(eff_kl * 100, 2), "%\n")
    #> Efficiency of uniform design: 44.11 %

## Example 2: exponential decay, Poisson family

Context. A drug clearance study where the event count follows a Poisson process with rate \(\mu(x) = \exp(a - bx)\). The reference model has nominal decay rate \(b = 0.5\). The rival represents a faster-elimination regime (\(b \in [0.8, 1.5]\)), which could arise from drug interactions. The KL divergence for Poisson is \(\text{KL} = \mu_2 - \mu_1 - \mu_1 \log(\mu_2/\mu_1)\); since the rival decays faster, the gap is largest at long follow-up times.

    result_kl_pois <- opt_des(
      "KL-Optimality",
      model        = y ~ exp(a - b * x),
      parameters   = c("a", "b"),
      par_values   = c(2, 0.5),
      design_space = c(0, 4),
      rival_pars   = c(2, 1.0),
      rival_lower  = c(1.5, 0.8),
      rival_upper  = c(2.5, 1.5),
      family       = "Poisson",
      phi          = 1
    )
    #>
    #> ℹ Stop condition reached: difference between sensitivity and criterion < 1e-05
    #> ⠙ Calculating optimal design 4 done (5.2/s) | 768msℹ The lower bound for efficiency is 99.9999938849073%
    #>
    result_kl_pois
    #> Optimal design for KL-Optimality:
    #>   Point    Weight
    #> 1     0 0.2012678
    #> 2     4 0.7987322
    summary(result_kl_pois)
    #> Model:
    #> y ~ exp(a - b * x)
    #> Weight function:
    #> function (x)
    #> 1
    #>
    #> GLM family: Poisson  (phi = 1)
    #> Optimal rival parameters: 2.2799, 0.8
    #>
    #> Optimal design for KL-Optimality:
    #>   Point    Weight
    #> 1     0 0.2012678
    #> 2     4 0.7987322
    #>
    #> Minimum efficiency (Atwood): 99.9999938849073%
    #> Criterion value:             0.318553

    plot(result_kl_pois)

KL sensitivity for Poisson decay model.

The optimal rival parameters found internally confirm the closest rival within the constraint:

    hv <- attr(result_kl_pois, "hidden_value")
    cat("Optimal rival: a =", round(hv$beta2_star[1], 3),
        " b =", round(hv$beta2_star[2], 3), "\n")
    #> Optimal rival: a = 2.28  b = 0.8

## Using `make_kl_fun()` for custom KL functions

For some discrimination problems the KL divergence does not reduce to the standard cumulant-generating-function form that `opt_des()` uses internally. `make_kl_fun()` automates the construction of the KL function from two model-family pairs.

### Supported pairs

Combination | Formula
--- | ---
Same family, same \(\phi\) | Standard cumulant form
Normal vs Normal, \(\phi_1 \neq \phi_2\) | \(\frac{1}{2}\left[\log\frac{\phi_2}{\phi_1} + \frac{\phi_1}{\phi_2} + \frac{(\mu_1-\mu_2)^2}{\phi_2} - 1\right]\)
Gamma vs Gamma, different shape | Closed form with digamma/lgamma

For other pairs, supply `kl_fun` directly as a function `(x, beta2) -> scalar`.

### Example 3: Normal with different variances (1D)

Context. Same exponential decay model, but now the rival has variance \(\phi_2 = 4\) (four times larger). The KL function includes a constant term \(\frac{1}{2}[\log(\phi_2/\phi_1) + \phi_1/\phi_2 - 1]\) even when the means coincide, so the design must also account for the variance difference.

    kl_fn_var <- make_kl_fun(
      "Normal",
      model1      = y ~ a * exp(-b * x),
      params1     = c("a", "b"),
      par_values1 = c(1, 0.5),
      phi1        = 1,
      family2     = "Normal",
      model2      = y ~ c * exp(-d * x),
      params2     = c("c", "d"),
      phi2        = 4
    )

    result_kl_var <- opt_des(
      "KL-Optimality",
      model        = y ~ a * exp(-b * x),
      parameters   = c("a", "b"),
      par_values   = c(1, 0.5),
      design_space = c(0, 4),
      kl_fun       = kl_fn_var,
      rival_pars   = c(1, 1.0),
      rival_lower  = c(0.5, 0.8),
      rival_upper  = c(2.0, 1.5)
    )
    #> ⠙ Calculating optimal design 7 done (3.1/s) | 2.2s⠹ Calculating optimal design 8 done (3.1/s) | 2.6s⠸ Calculating optimal design 9 done (3/s) | 3s    ⠼ Calculating optimal design 10 done (3/s) | 3.3s⠴ Calculating optimal design 11 done (3/s) | 3.7s⠦ Calculating optimal design 12 done (3/s) | 4.1s⠧ Calculating optimal design 13 done (2.9/s) | 4.4s⠇ Calculating optimal design 14 done (2.9/s) | 4.8s⠏ Calculating optimal design 15 done (2.9/s) | 5.2s⠋ Calculating optimal design 16 done (2.9/s) | 5.5s⠙ Calculating optimal design 17 done (2.9/s) | 5.9s⠹ Calculating optimal design 18 done (2.9/s) | 6.2s⠸ Calculating optimal design 19 done (2.9/s) | 6.5s⠼ Calculating optimal design 20 done (2.9/s) | 6.9s⠴ Calculating optimal design 21 done (2.9/s) | 7.2s
    #> ℹ Stop condition not reached, max iterations performed
    #> ⠦ Calculating optimal design 22 done (2.9/s) | 7.5sℹ The lower bound for efficiency is 99.9671378206639%
    #>
    result_kl_var
    #> Optimal design for KL-Optimality:
    #>      Point     Weight
    #> 1 0.000000 0.16826906
    #> 2 1.333333 0.07912872
    #> 3 1.989481 0.61240029
    #> 4 2.666667 0.14020193
    summary(result_kl_var)
    #> Model:
    #> y ~ a * exp(-b * x)
    #> Weight function:
    #> function (x)
    #> 1
    #>
    #> KL function: user-supplied
    #> Optimal rival parameters: 1.1353, 0.8
    #>
    #> Optimal design for KL-Optimality:
    #>      Point     Weight
    #> 1 0.000000 0.16826906
    #> 2 1.333333 0.07912872
    #> 3 1.989481 0.61240029
    #> 4 2.666667 0.14020193
    #>
    #> Minimum efficiency (Atwood): 99.9671378206639%
    #> Criterion value:             0.320445

    plot(result_kl_var)

KL sensitivity: Normal phi=1 vs Normal phi=4.

### Example 4: two-factor MM vs linear rival (`make_kl_fun`, 2D)

`make_kl_fun()` works with models that use different subsets of the design variables. Here the reference uses \((x_1, x_2)\) (bisubstrate MM) while the rival depends only on \(x_1\) (single-substrate linear approximation):

    kl_fn_2d <- make_kl_fun(
      "Normal",
      model1      = y ~ Vmax * x1 * x2 / ((Km1 + x1) * (Km2 + x2)),
      params1     = c("Vmax", "Km1", "Km2"),
      par_values1 = c(1, 1, 1),
      model2      = y ~ alpha * x1,
      params2     = "alpha"
    )

    result_kl_2d <- opt_des(
      "KL-Optimality",
      model        = y ~ Vmax * x1 * x2 / ((Km1 + x1) * (Km2 + x2)),
      parameters   = c("Vmax", "Km1", "Km2"),
      par_values   = c(1, 1, 1),
      design_space = list(x1 = c(0.1, 5), x2 = c(0.1, 5)),
      kl_fun       = kl_fn_2d,
      rival_pars   = c(0.5),
      rival_lower  = c(0.05),
      rival_upper  = c(2.0)
    )
    #>
    #> ℹ Stop condition reached: difference between sensitivity and criterion < 1e-05
    #> ⠙ Calculating optimal design 9 done (5.4/s) | 1.7sℹ The lower bound for efficiency is 99.9887728054157%
    #>
    result_kl_2d
    #> Optimal design for KL-Optimality (2 factors):
    #>         x1       x2    Weight
    #> 1 2.048054 4.998983 0.7094157
    #> 2 5.000000 0.100000 0.2905843

The heatmap concentrates support where the difference between the bisubstrate hyperbola and the linear rival is greatest (saturation regime in both \(x_1\) and \(x_2\)):

    plot(result_kl_2d)

KL sensitivity heatmap: 2D MM vs linear rival.

## Example 5: discriminating error structures — the Hill model

Reference: De la Calle-Arroyo, Leorato, Rodríguez-Aragón & Tommasi (2025). Augmented designs to choose between constant absolute and relative errors. Chemometrics and Intelligent Laboratory Systems, doi:10.1016/j.chemolab.2025.105374.

Context. Dose-response experiments with the Hill (4-parameter Emax) model:

\[\eta(x, \beta) = (E_{\text{con}} - b) \cdot \frac{(x/\text{IC}_{50})^s}{1 + (x/\text{IC}_{50})^s} + b\]

with \(E_{\text{con}} = 1.70\), \(b = 0.137\), \(\text{IC}_{50} = 111\), \(s = -1.03\) and \(x \in [0.01, 1500]\) nM. Two error structures are competing:

- Relative error (reference): \(\varepsilon \sim N(0,\, \sigma_{\text{rel}}^2 \cdot \eta^2)\)
- Absolute error (rival): \(\varepsilon \sim N(0,\, \sigma_{\text{abs}}^2)\)

The KL divergence minimised over the rival variance \(\sigma_{\text{abs}}^2\) is

\[\text{KL}(x,\, \sigma_{\text{abs}}^2) = \frac{1}{2}\!\left(\frac{\eta^2(x)}{\sigma_{\text{abs}}^2} - 1 + \log\frac{\sigma_{\text{abs}}^2}{\eta^2(x)}\right).\]

We supply this directly as `kl_fun` since it is not covered by the standard Normal-vs-Normal formula (the reference variance is \(x\)-dependent):

    kl_fun_hill <- function(x, beta2) {
      sigma_sq <- beta2[1]
      eta <- (1.70 - 0.137) * (x / 111)^(-1.03) / (1 + (x / 111)^(-1.03)) + 0.137
      0.5 * (eta^2 / sigma_sq - 1 + log(sigma_sq / eta^2))
    }

    result_kl_hill <- opt_des(
      "KL-Optimality",
      model        = y ~ (Econ - b) * (x / IC50)^s / (1 + (x / IC50)^s) + b,
      parameters   = c("Econ", "b", "IC50", "s"),
      par_values   = c(1.70, 0.137, 111, -1.03),
      design_space = c(0.01, 1500),
      kl_fun       = kl_fun_hill,
      rival_pars   = c(1.0),
      rival_lower  = c(1e-4),
      rival_upper  = c(1e6)
    )
    #>
    #> ℹ Stop condition reached: difference between sensitivity and criterion < 1e-05
    #> ⠙ Calculating optimal design 5 done (21/s) | 238msℹ The lower bound for efficiency is 99.9997190548208%
    #>
    result_kl_hill
    #> Optimal design for KL-Optimality:
    #>     Point   Weight
    #> 1    0.01 0.233994
    #> 2 1500.00 0.766006
    summary(result_kl_hill)
    #> Model:
    #> y ~ (Econ - b) * (x/IC50)^s/(1 + (x/IC50)^s) + b
    #> Weight function:
    #> function (x)
    #> 1
    #>
    #> KL function: user-supplied
    #> Optimal rival parameters: 0.7192
    #>
    #> Optimal design for KL-Optimality:
    #>     Point   Weight
    #> 1    0.01 0.233994
    #> 2 1500.00 0.766006
    #>
    #> Minimum efficiency (Atwood): 99.9997190548208%
    #> Criterion value:             0.813492

    plot(result_kl_hill)

KL sensitivity for the Hill model (error structure discrimination).

The KL-optimal design places all weight at the two extremes of the design space. This matches the result reported in the paper (support at \(x = 0.01\) with weight 0.23 and \(x = 1500\) with weight 0.77). The Atwood criterion at 100 % confirms the design is exactly optimal.

The optimal rival variance recovered internally:

    hv_hill <- attr(result_kl_hill, "hidden_value")
    cat("Optimal rival sigma_abs^2 =", round(hv_hill$beta2_star, 4), "\n")
    #> Optimal rival sigma_abs^2 = 0.7192

Note that with only two support points this design cannot estimate all four parameters of the Hill model. The paper proposes combining it with a D-optimal design via the augmentation workflow described in `vignette("optedr-augment")`.