gformula: Estimating causal effects in the presence of time-varying confounding or mediation using the g-computation formula

This article describes a new command, gformula, that is an implementation of the g-computation procedure. It is used to estimate the causal effect of time-varying exposures on an outcome in the presence of time-varying confounders that are themselves also affected by the exposures. The procedure also addresses the related problem of estimating direct and indirect effects when the causal ef- fect of the exposures on an outcome is mediated by intermediate variables, and in particular when confounders of the mediator–outcome relationships are them- selves affected by the exposures. A brief overview of the theory and a description of the command and its options are given, and illustrations using two simulated examples are provided. Copyright 2011 by StataCorp LP.