Performance of propensity score matching to estimate causal effects in small samples

Propensity score (PS) matching is a very popular causal estimator usually used to estimate the average treatment effect on the treated (ATT) from observational data. However, opting for this estimator may raise some efficiency issues when the sample size is limited. Therefore, we aimed to evaluate the performance of propensity score matching in this context. We started with a motivating example based on a cohort of 66 children with sickle cell anemia who received either allogeneic bone-marrow transplant or chronic transfusion. We found substantial differences in the ATT estimate according to the model selected for propensity score estimation and subsequent matching. Then, we assessed the performance of the different propensity score matching methods and post-matching analyses to estimate the ATT using a simulation study. Although all selected propensity score matching methods were based of previous recommendations, we found important discrepancies in the estimation of treatment effect between them, underlining the importance of thorough sensitivity analyses when using propensity score matching in the context of small sample sizes.