A Tutorial on Causal Inference in Longitudinal Data With Time-Varying Confounding Using G-Estimation

In psychological research, longitudinal study designs are often used to examine the effects of a naturally observed predictor (i.e., treatment) on an outcome over time. But causal inference of longitudinal data in the presence of time-varying confounding is notoriously challenging. In this tutorial, we introduce g-estimation, a well-established estimation strategy from the causal inference literature. G-estimation is a powerful analytic tool designed to handle time-varying confounding variables affected by treatment. We offer step-by-step guidance on implementing the g-estimation method using standard parametric regression functions familiar to psychological researchers and commonly available in statistical software. To facilitate hands-on usage, we provide software code at each step using the open-source statistical software R. All the R code presented in this tutorial are publicly available online.