These data arise in several areas of investigation and are characterized by the presence of correlation between observations within the same group.
Some examples are repeated measures data, longitudinal studies, and nested designs.
Classical modeling techniques which assume independence of the observations are not appropriate for grouped data. Some features of NLME 3.3 Maximum likelihood and restricted maximum likelihood estimation Multilevel models for data with multiple nested grouping levels. These incorporate novel computational methods, described in the Bell Labs Technical Memorandum Computational Methods for Multilevel Models , available available here as PostScript or as Portable Document Format . Methods for plotting, summarizing, and fitting groupedData objects are also included. The plot methods for fitted objects include a formula argument which gives them unlimited flexibility for generating diagnostic plots.