getX {semEff}

Get Model Design Matrix

Description

Return the design matrix for a fitted model, with some additional options.

Usage

getX(
  mod,
  data = NULL,
  contrasts = NULL,
  add.data = FALSE,
  centre = FALSE,
  scale = FALSE,
  as.df = FALSE,
  merge = FALSE,
  env = NULL
)

Arguments

mod

A fitted model object, or a list or nested list of such objects. Can also be a model formula(s) or character vector(s) of term names (in which case data must be supplied).

data

An optional dataset, used to refit the model(s) and/or construct the design matrix.

contrasts

Optional, a named list of contrasts to apply to factors (see the contrasts.arg argument of model.matrix() for specification). These will override any existing contrasts in the data or model call.

add.data

Logical, whether to append data not specified in the model formula (with factors converted to dummy variables).

centre, scale

Logical, whether to mean-centre and/or scale terms by standard deviations (for interactions, this is carried out prior to construction of product terms). Alternatively, a numeric vector of means/standard deviations (or other statistics) can be supplied, whose names must match term names.

as.df

Logical, whether to return the matrix as a data frame (without modifying names).

merge

Logical. If TRUE, and mod is a list or nested list, a single matrix containing all terms is returned (variables must be the same length).

env

Environment in which to look for model data (if none supplied). Defaults to the formula() environment.

Details

This is primarily a convenience function to enable more flexible construction of design matrices, usually for internal use and for further processing. Use cases include processing and/or return of terms which may not be present in a typical design matrix (e.g. constituents of product terms, dummy variables).

Value

A matrix or data frame of model(s) terms, or a list or nested list of same.

See Also

model.matrix()

Examples

# Model design matrix (original)
m <- shipley.growth[[3]]
x1 <- model.matrix(m)
x2 <- getX(m)
stopifnot(all.equal(x1, x2, check.attributes = FALSE))

# Using formula or term names (supply data)
d <- shipley
x1 <- getX(formula(m), data = d)
x2 <- getX(names(lme4::fixef(m)), data = d)
stopifnot(all.equal(x1, x2))

# Scaled terms
head(getX(m, centre = TRUE, scale = TRUE))

# Combined matrix for SEM
head(getX(shipley.sem, merge = TRUE))
head(getX(shipley.sem, merge = TRUE, add.data = TRUE))  # add other variables