Interpretation of Subgroup Effects in Published Trials

With the rapidly expanding number of studies reporting on treatment subgroups come new challenges in analyzing and interpreting this sometimes complex area of the literature. This article discusses 3 important issues regarding the analysis and interpretation of existing trials or systematic reviews that report on treatment effect modifiers (subgroups) for specific physical therapy interventions. The key messages are: (1) point estimates of treatment modifier effect size (interaction effect) and their confidence intervals can be calculated using group-level data when individual patient-level data are not available; (2) interaction effects do not define the total effect size of the intervention in the subgroup but rather how much more effective it is in the subgroup than in those not in the subgroup; (3) recommendations regarding the use of an intervention in a subgroup need to consider the size and direction of the main effect and the interaction effect; and (4) rather than simply judging whether a treatment modifier effect is clinically important based only on the interaction effect size, a better criterion is to determine whether the combined effect of the interaction effect and main effect makes the difference between an overall effect that is clinically important and one that is not clinically important.