Expression (CovariateModel) - Define relationship between parameters and covariates

Description

The Expression property is a string or cell array of strings, where each string represents the relationship between a parameter and one or more covariates. The Expression property denotes fixed effects with the prefix theta, and random effects with the prefix eta.

Each expression string must be in the form:

parameterName = relationship

This example of an expression string defines the relationship between a parameter (volume) and a covariate (weight), with fixed effects, but no random effects:

CovModelObj.Expression = {'volume = theta1 + theta2*weight'};

This table illustrates expression formats for some common parameter-covariate relationships.

Parameter-Covariate Relationship	Expression Format
Linear with random effect	Cl = theta1 + theta2*WEIGHT + eta1
Exponential without random effect	Cl = exp(theta_Cl + theta_Cl_WT*WEIGHT)
Exponential, WEIGHT centered by mean, has random effect	Cl = exp(theta1 + theta2*(WEIGHT - mean(WEIGHT)) + eta1)
Exponential, log(WEIGHT), which is equivalent to power model	Cl = exp(theta1 + theta2*log(WEIGHT) + eta1)
Exponential, dependent on WEIGHT and AGE, has random effect	Cl = exp(theta1 + theta2*WEIGHT + theta3*AGE + eta1)

Tip   To simultaneously fit data from multiple dose levels, use a CovariateModel object as an input argument to sbionlmefit, and omit the random effect (eta) from the Expression property in the CovariateModel object.

The Expression property must meet the following requirements:

• The expression strings are valid MATLAB code.

• Each expression string is linear with a transformation.

• There is exactly one expression string for each parameter.

• In each expression string, a covariate is used in at most one term.

• In each expression string, there is at most one random effect (eta)

• Fixed effect (theta) and random effect (eta) names are unique within and across expression strings. That is, each covariate has its own fixed effect.

Tip   Use the getCovariateData method to view the covariate data when writing equations for the Expression property of a CovariateModel object.

Tip   Use the verify method to check that the Expression property of a CovariateModel object meets the conditions described previously.

Characteristics

Applies to	Object: CovariateModel
Data type	String or cell array of strings
Data values	parameterName = relationship
Access	Read/write

See Also

CovariateModel | getCovariateData | verify

How To

• Modeling the Population Pharmacokinetics of Phenobarbital in Neonates
• Specifying a Covariate Model

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