Constructs a character representation of the random effects formula
for fitting a MMRM for subject by visit in the format required for mmrm::mmrm().
random_effects_expr(
cov_struct = c("us", "ad", "adh", "ar1", "ar1h", "cs", "csh", "toep", "toeph"),
cov_by_group = FALSE
)Character - The covariance structure to be used, must be one of "us" (default),
"ad", "adh", "ar1", "ar1h", "cs", "csh", "toep", or "toeph")
Boolean - Whenever or not to use separate covariances per each group level
For example assuming the user specified a covariance structure of "us" and that no groups were provided this will return
us(visit | subjid)
If cov_by_group is set to FALSE then this indicates that separate covariance matrices
are required per group and as such the following will be returned:
us( visit | group / subjid )