Statistical models appropriate for designs often used in group-randomized trials.

Group-randomized trials are characterised by the allocation of identifiable groups rather than individuals to study conditions; members within those groups are then observed to assess the effect of the intervention. It is convenient to categorize the designs employed in group-randomized trials along two dimensions, each with two levels. The first distinguishes between designs having just one or two time intervals and those having three or more intervals. The second distinguishes between nested cohort and nested cross-sectional designs. Following a brief review of the design and analytic issues common to group-randomized trials, and their general solutions, this paper presents the adaptations of the mixed-model analysis of covariance and random coefficients models that are required for the four combinations that result from this categorization scheme. The assumptions, strengths and weaknesses of each model are discussed. Copyright 2001 John Wiley & Sons, Ltd.