Methods and Applications of Statistics in Clinical Trials, Volume 1: Concepts, Principles, Trials, and Designs

Chapter 36

Group-Randomized Trials

David M. Murray

36.1 Introduction

Group-randomized trials (GRTs) are comparative studies used to evaluate interventions that operate at a group level, manipulate the physical or social environment, or cannot be delivered to individuals [1]. Examples include school-, worksite-, and community-based studies designed to improve the health of students, employees, or residents.

Four characteristics distinguish the GRT from the more familiar randomized clinical trial (RCT) [1]. First, the unit of assignment is an identifiable group; such groups are not formed at random, but rather through some physical, social, geographic, or other connection among their members. Second, different groups are assigned to each condition, creating a nested or hierarchical structure for the design and the data. Third, the units of observation are members of those groups so that they are nested within both their condition and their group. Fourth, only a limited number of groups assigned to each condition usually exists.

Together, these characteristics create several problems for the design and analysis of GRTs. The major design problem is that a limited number of often heterogeneous groups makes it difficult for randomization to distribute potential sources of confounding evenly in any single realization of the experiment, which increases the need to employ design strategies that will limit confounding and analytic strategies to deal with confounding where it is detected. ...