The estimated coefficients at level $i$ are obtained by adding
together the fixed effects estimates and the corresponding random
effects estimates at grouping levels less or equal to $i$. The
resulting estimates are returned as a data frame, with rows
corresponding to groups and columns to coefficients. Optionally, the
returned data frame may be augmented with covariates summarized over
groups.
a data frame inheriting from class coef.lme with the estimated
coefficients at level level and, optionally, other covariates
summarized over groups. The returned object also inherits from classes
ranef.lme and data.frame.