Get data_sigma_group
# S3 method for pk
get_data_sigma_group(obj, newdata = NULL, ...)
A `factor` vector giving the error SD group ID for each observation, as the interaction of the factors specified in `obj$stat_error_model$error_group`.
A [pk()] object
Optional: A `data.frame` with new data for which to get the `data_sigma_group`s. If NULL (the default), then the groups will be evaluated for the `obj$data`.
Additional arguments. Not currently in use.
Caroline Ring