Risk for marijuana

Marijuana is the most commonly used illicit drug in the U.S. Approximately 46% of college students report having tried marijuana, 27% report use in the past year, and 16% report past 30-day use (1); thus, a significant proportion of college students use marijuana. Furthermore, problems associated with use are, unfortunately, not uncommon. For example, short-term cognitive impairments and impairment in educational performance have been associated with heavy marijuana use (2-4). Given this, identification of individuals who use marijuana and report associated problems is an important issue. However, despite the relative prevalence of use, many college students have never tried marijuana or do not currently use it. As a result, an assessment of marijuana-related problems will yield a mixed distribution with a high number of zero-values. Some respondents will report not experiencing problems because they in fact did not use marijuana at all during the assessment window and those who do use the drug will report a range of problems including, for some users, zero. Identifying variables that predict nonusers as well as identifying predictors of the number of problems experienced among users are both of interest. Zero-inflated Poisson (ZIP; e.g., Lambert (5)) and a subsequent generalization of ZIP, Zero-inflated negative binomial regression (ZINB; e.g., Heilbron (6)) are two statistical techniques that allow one to accomplish both of these objectives in a single analysis. They, thus, represent parsimonious procedures that allow one to examine effects over the full distribution. These mixed distributions are a common feature in research investigating risk behaviors with low base rates (6).

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