Examining moderation analyses in propensity score methods: Application to depression and substance use.

OBJECTIVE: This study provides guidance on how propensity score methods can be combined with moderation analyses (i.e., effect modification) to examine subgroup differences in potential causal effects in nonexperimental studies. As a motivating example, we focus on how depression may affect subsequent substance use differently for men and women. METHOD: Using data from a longitudinal community cohort study (N = 952) of urban African Americans with assessments in childhood, adolescence, young adulthood, and midlife, we estimate the influence of depression by young adulthood on substance use outcomes in midlife, and whether that influence varies by gender. We illustrate and compare 5 different techniques for estimating subgroup effects using propensity score methods, including separate propensity score models and matching for men and women, a joint propensity score model for men and women with matching separately and together by gender, and a joint male/female propensity score model that includes theoretically important gender interactions with matching separately and together by gender. RESULTS: Analyses showed that estimating separate models for men and women yielded the best balance and, therefore, is a preferred technique when subgroup analyses are of interest, at least in this data. RESULTS also showed substance use consequences of depression but no significant gender differences. CONCLUSIONS: It is critical to prespecify subgroup effects before the estimation of propensity scores and to check balance within subgroups regardless of the type of propensity score model used. RESULTS also suggest that depression may affect multiple substance use outcomes in midlife for both men and women relatively equally.