Modeling with constraints"
In flexFitR: Flexible Non-Linear Least Square Model Fitting

knitr::opts_chunk$set(
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This vignette demonstrates how to apply parameter constraints when modeling biological processes using {flexFitR}. Constraints can help ensure that parameter estimates remain within realistic or biologically meaningful ranges, improving both the interpretability and reliability of model outcomes.

Introduction to Modeling with Constraints

In many biological models, certain relationships between parameters are expected. For example:

Some parameters should not exceed certain values (e.g., maximum growth rates).
Some parameters should maintain specific relationships with each other (e.g., one stage occurring before another in time).

This vignette demonstrates how to apply these types of constraints in {flexFitR} to guide the optimization process.

Example Case

For this example, we use the Green Leaf Index (GLI) derived from UAV imagery to model plant emergence, canopy closure, and senescence. The parameters we are interested in include:

t1: Emergence time
t2: Canopy closure time
t3: Senescence onset

Our expectation is that $0 < t1 < t2 < t3$. We will apply constraints to ensure this relationship hold.

Loading libraries

library(flexFitR)
library(dplyr)
library(kableExtra)
library(ggpubr)
library(purrr)

1. Exploring data

We begin with the explorer function, which provides basic statistical summaries and visualizations to help understand the temporal evolution of each plot.

data(dt_potato)
explorer <- explorer(dt_potato, x = DAP, y = c(GLI), id = Plot)

p1 <- plot(explorer, type = "evolution", return_gg = TRUE, add_avg = TRUE)
p2 <- plot(explorer, type = "x_by_var", return_gg = TRUE)
ggarrange(p1, p2, nrow = 1)

kable(mutate_if(explorer$summ_vars, is.numeric, round, 2))

2. Regression function

After exploring the data, we define the regression function. Here we use a linear-plateau-linear function with five parameters: t1, t2, t3, k, and $\beta$. The function can be expressed mathematically as follows:

fn_lin_pl_lin()

\begin{equation} f(t; t_1, t_2, t_3, k, \beta) = \begin{cases} 0 & \text{if } t < t_1 \ \dfrac{k}{t_2 - t_1} \cdot (t - t_1) & \text{if } t_1 \leq t \leq t_2 \ k & \text{if } t_2 \leq t \leq t_3 \ k + \beta \cdot (t - t_3) & \text{if } t > t_3 \end{cases} \end{equation}

plot_fn(
  fn = "fn_lin_pl_lin",
  params = c(t1 = 38.7, t2 = 62, t3 = 90, k = 0.32, beta = -0.01),
  interval = c(0, 108),
  color = "black",
  base_size = 15
)

To impose constraints, we can reformulate the function. For instance, if we want to ensure that $t3 \geq t2$, we introduce dt as the difference between t3 and t2:

\begin{equation} f(t; t_1, t_2, dt, k, \beta) = \begin{cases} 0 & \text{if } t < t_1 \ \dfrac{k}{t_2 - t_1} \cdot (t - t_1) & \text{if } t_1 \leq t \leq t_2 \ k & \text{if } t_2 \leq t \leq (t_2 + dt) \ k + \beta \cdot (t - (t_2 + dt)) & \text{if } t > (t_2 + dt) \end{cases} \end{equation}

To enforce $dt > 0$ and $\beta < 0$ (i.e., a non-positive slope at the end of the curve), we specify bounds in the modeler function as follows:

# Define constraints and bounds for the model
lower_bounds <- c(t1 = 0, t2 = 0, dt = 0, k = 0, beta = -Inf)
upper_bounds <- c(t1 = Inf, t2 = Inf, dt = Inf, k = Inf, beta = 0)
# Initial values
initial_vals <- c(t1 = 38, t2 = 62, dt = 28, k = 0.32, beta = -0.01)

3. Fitting Models with Constraints

We fit the model with these constraints by passing lower and upper arguments to modeler. In this vignette, we fit the model for plots 195 and 40 as a subset of the total 196 plots.

mod_1 <- dt_potato |>
  modeler(
    x = DAP,
    y = GLI,
    grp = Plot,
    fn = "fn_lin_pl_lin2",
    parameters = initial_vals,
    lower = lower_bounds,
    upper = upper_bounds,
    method = c("nlminb", "L-BFGS-B"),
    subset = c(195, 40)
  )

Here:

x specifies the days after planting (DAP),
y is the GLI variable to be modeled
grp enables group analysis across multiple plots
parameters are the initial parameter values
method specifies the optimization methods to evaluate

After fitting, we can inspect the model summary and visualize the fit using the plot function:

print(mod_1)

plot(mod_1, id = c(195, 40))
kable(mod_1$param)

3.1. Extracting model coefficients and uncertainty measures

Once the model is fitted, we can extract key statistical information, such as coefficients, standard errors, confidence intervals, and the variance-covariance matrix for each plot. These metrics help evaluate parameter reliability and assess uncertainty.

The functions coef, confint, and vcov are used as follows:

coef: Extracts the estimated coefficients for each group.
confint: Provides the confidence intervals for the parameter estimates.
vcov: Returns the variance-covariance matrix, which can be used to understand the relationships between the estimates and their variability.

coef(mod_1, id = 40)

confint(mod_1, id = 40)

vcov(mod_1, id = 40)

4. Plotting options

Using type = 2 in the plot function generates a coefficients plot. This allows us to view the estimated coefficients and their associated confidence intervals for each group.

plot(mod_1, type = 2, id = c(195, 40), label_size = 8)

Another option (type = 4) shows the fitted curve (black line), confidence interval (blue-dashed line), and prediction interval (red-dashed line). Additionally, setting type = 5 displays the first derivative, indicating the rate of change over time.

a <- plot(mod_1, type = 4, color = "black", title = "Fitted Curve + CIs & PIs")
b <- plot(mod_1, type = 5, color = "black")
ggarrange(a, b)

5. Conclusion

This vignette showed how to apply constraints in {flexFitR} models to better capture biological realities and improve parameter estimation. Constraints can be an essential tool for ensuring that models produce interpretable and meaningful results.