Mediation analysis with zero-inflated substance use outcomes: Challenges and recommendations

Mediating mechanisms are important components of substance use research, as many substance use interventions work by targeting mediating variables. One issue that is common in substance use research is the presence of many responses of zero in a count variable that is the primary outcome of interest, such as number of drinks per week or number of substances used in the past month. The goal of this paper is to highlight the unique challenges that substance use researchers face when conducting mediation analysis with a zero-inflated count outcome. In this paper, we first describe the models that are commonly used for zero-inflated count data, and when it is appropriate to use them. We then describe general mediation analysis and summarize the small body of work that has focused on mediation for count and zero-inflated count outcomes. We identify the main issue of computing the mediated effect when outcomes are zero-inflated, namely, that the path leading to the zero-inflated count outcome (or mediator)is modeled in two parts. We then provide two examples of mediation models with different conclusions that have zero-inflated count outcomes using adolescent substance use data and define the issues that arise when assessing mediation for each. Finally, we describe the directions in which we must target future methodological research to create accessible solutions for handling mediation with zero-inflated count data in substance use research.