Introduction: model structure for Simulx
The code in the Model tab of Simulx contains the [LONGITUDINAL] section, but can also contain the [COVARIATE] and [INDIVIDUAL] sections to describe the statistical part of the model.
[LONGITUDINAL]: structural model and error model
[INDIVIDUAL]: parameter distributions
[COVARIATE]: covariate declaration and transformations
Depending on the cases, these sections are automatically created or should be provided by the user:
When a Monolix project is imported into Simulx, the model with all sections is created automatically from the .txt Monolix model file and .mlxtran Monolix project file.
When a PKanalix project is imported into Simulx, the [LONGITUDINAL] section is created automatically from the .txt PKanalix model file. It is possible to then manually add [COVARIATE] and [INDIVIDUAL] sections to consider covariates and inter-individual variability in the simulations.
When starting a Simulx project from scratch, the [LONGITUDINAL] section is mandatory and the [COVARIATE] and [INDIVIDUAL] sections are necessary if covariates and inter-individual variability are present in the model.
More information is given in Setting the model about how the model is saved in when starting a Simulx project from scratch or after importing a Monolix or PKanalix project.
The sections are themselves divided into blocks, which allow to define different types of variables in different ways:
DESCRIPTION: (optional): block to write comments on the model
input (mandatory): list of input parameters
EQUATION: (optional): block were ODEs can be defined, as well as mathematical expressions (e.g parameter transformations)
DEFINITION: (optional): block to define random variables (discrete or continuous) via their probability distribution
within the [LONGITUDINAL] section: error model, probability distribution for count/categorical output and hazard for time-to-event output
within the [INDIVIDUAL] section: distribution of the parameters
OUTPUT: (mandatory): block indicating the output of the model
output (mandatory): list of output parameters included in the OUTPUT block.
Below we show the possible sections and blocks in a Simulx model:
