Using Propensity Scores with Small Samples

Propensity scores are increasingly being used in large sample studies to control for pregroup differences. Because these scores are often used to match cases, they can result in sample attrition. In smaller sample studies, such attrition leaves too few cases for meaningful analysis. Alternatives when working with small samples are to use propensity scores as covariates to control for pre-group differences and to use propensities as weights in weighted regression. The use of propensity scores with small samples is compared with the alternative of using baseline measures to control for pre-group differences. The paper also presents a procedure for empirically testing whether construct integrity holds. We use data from a dosage specific study of substance abusing families receiving clinical services and coordinated case management. Program outcomes are examined, comparing the use of propensity scores with the use of time one measures alone. Results show that propensities can be used as covariates or as weights in small samples and produce more reliable results than matching procedures. Using time one measures as a control produces nearly as good results.