Function used to set up a basic grouping term in brms.
The function does not evaluate its arguments --
it exists purely to help set up a model with grouping terms.
gr is called implicitly inside the package
and there is usually no need to call it directly.
gr(..., by = NULL, cor = TRUE, id = NA, cov = NULL, dist = "gaussian")One or more terms containing grouping factors.
An optional factor variable, specifying sub-populations of the
groups. For each level of the by variable, a separate
variance-covariance matrix will be fitted. Levels of the grouping factor
must be nested in levels of the by variable.
Logical. If TRUE (the default), group-level terms will be
modelled as correlated.
Optional character string. All group-level terms across the model
with the same id will be modeled as correlated (if cor is
TRUE). See brmsformula for more details.
An optional matrix which is proportional to the withon-group
covariance matrix of the group-level effects. All levels of the grouping
factor should appear as rownames of the corresponding matrix. This argument
can be used, among others, to model pedigrees and phylogenetic effects. See
vignette("brms_phylogenetics") for more details. By default, levels
of the same grouping factor are modeled as independent of each other.
Name of the distribution of the group-level effects.
Currently "gaussian" is the only option.
if (FALSE) {
# model using basic lme4-style formula
fit1 <- brm(count ~ Trt + (1|patient), data = epilepsy)
summary(fit1)
# equivalent model using 'gr' which is called anyway internally
fit2 <- brm(count ~ Trt + (1|gr(patient)), data = epilepsy)
summary(fit2)
# include Trt as a by variable
fit3 <- brm(count ~ Trt + (1|gr(patient, by = Trt)), data = epilepsy)
summary(fit3)
}