Stata tip 146: Using margins after a Poisson regression model to estimate the number of events prevented by an intervention

Abstract

After fitting a Poisson regression model to evaluate the effect of an intervention in a cohort study, one might be interested in estimating the number of events prevented by the intervention (assuming the observed associations are causal). This can be derived as the difference in the intervention group between the predicted number of events under the counterfactual (no intervention) and the factual (intervention) scenarios. One could use the predict command to obtain the predicted number of events under the two scenarios and then sum up the differences, but this approach would not be conve-nient for several reasons. One would need to change the intervention variable to get the counterfactual predicted values, and the confidence intervals would not be readily available ( bootstrap or jackknife could be used, but this could be particularly time consuming if the dataset is large). We here suggest the margins command. Its use, however, is not straight-forward for our specific problem margins computes observation then the these