The principal aim of the book is an exposition of methods for the analysis of clustered observations; the secondary one is to provide substantive interest as a measure of uncertainty, quality, equity, or generally, as a summary of differences among experimental or observational units. Another goal is to make a balanced presentation of the advantages and limitations of these methods The examples used for illustration of methods are not drawn exclusively from the social sciences. Although models are motivated mainly by social science problems, they are applicable in a variety of situations involving (imperfect) replication, such as repeated measurements, repeated experiments, logitudinal analysis, and analysis of covariance structures in general.