When is the Story in the Subgroups?

This paper examines strategies for interpreting and reporting estimates of intervention effects for subgroups of a study sample. The paper considers: why and how subgroup findings are important for applied research, alternative ways to define subgroups, different research questions that motivate subgroup analyses, the importance of pre-specifying subgroups before analyses are conducted, the importance of using existing theory and prior research to distinguish between subgroups for whom study findings are confirmatory (hypothesis testing) as opposed to exploratory (hypothesis generating), and the conditions under which study findings should be considered confirmatory. Each issue is illustrated by selected empirical examples.