Clinical trials conducted of new drugs prior to regulatory approval are usually categorized as Phase I, Phase II or Phase III. Phase I trials are almost always conducted at a single investigational site. Many Phase II trials are conducted at two or more investigational sites or centers. Virtually all Phase III clinical trials have to be conducted at several investigational sites in order to accrue the number of patients required by the protocol. Statistical analyses of multicenter clinical trials pose challenges in terms of what are the most appropriate analyses of the data collected. Chapter 8 of Peace and Chen (2010) discuss the importance of planning, conducting and statistically analyzing multicenter clinical trials. Analysis methods in that chapter are restricted to the usual linear models appropriate for a completely randomized block (centers) design (CRBD).