twopm: Two-part models

Abstract

In this article, we describe twopm, a command for fitting two-part models for mixed discrete-continuous outcomes. In the two-part model, a binary choice model is fit for the probability of observing a positive-versus-zero outcome. Then, conditional on a positive outcome, an appropriate regression model is fit for the positive outcome. The twopm command allows the user to leverage the capabilities of predict and margins to calculate predictions and marginal effects and their standard errors from the combined first- and second-part models.