Extracting model matrices
This function extracts the fixed-effect design matrix, random-effect design matrix, or the list of variance-covariance matrices whose weighted sum being the variance-covariance matrix of the response variable.
A character vector (only the first element will be used) specifying what kind of design matrix is requested. See details.
"X" are equivalent, requesting the fixed effect design matrix.
"Z" are equivalent, requesting the random effect design matrix. Note that this is an equivalent version of the design matrices such that the
tcrossprod will be the contribution to the marginal variance-covariance. This is not necessarily the one computed directly from the
random argument passed to
varComp. These are actually computed from
cholRoot of K matrices.
"K" are equivalent, requesting the contribution of each random effect to the marginal correlation matrix. These are not necessarily the same value passed to the
varcov argument of
varComp, because the input value will be treated as the "G" matrices when
random is not missing, but the result here will always be "K" matrices. See
varComp for notations.
"X", a single numeric matrix of fixed-effect design matrix.
Otherwise, a list of requested matrices.
See details on possible confusions.
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