Speaker: Dr. Dave Flora, York University
Department of Psychology

Title: Two-part Models for Semicontinuous Variables in Psychological Research

Abstract: “Semicontinuous” outcome variables arise regularly in psychological research, such as substance use research and developmental psychopathology, and applications in experimental psychology are also possible (e.g., response-time data). Such variables are characterized by a strictly non-negative continuous distribution coupled with a high frequency of observations equal to zero. Although models for zero-inflated count data are relatively well understood, models for continuous outcomes with a preponderance of zeros are less commonly used. I will describe both cross-sectional and longitudinal two-part models for such variables. Part 1 is a model for a binary outcome (whether a zero is observed) while Part 2 is a model for a continuous outcome, given that a non-zero value was observed in the first part. In the longitudinal case, model choice for Part 1 has implications for the interpretation of parameters in Part 2. These models will be illustrated with an application to adolescent alcohol use.