Medical marijuana laws, substance use treatment admissions and the ecological fallacy

Abstract

Sir: Meinhofer et al. 1 analyzes administrative data from state medical marijuana laws and substance use treatment admissions in an ecological study design to show that medical marijuana laws are associated with statelevel increases in marijuana, alcohol and cocaine treatment admissions among pregnant women. While these findings are provocative, the utility of ecological analyses in assessing the effects of medical marijuana use is severely limited and this study's results could reflect bias, rather than an individuallevel effect. Ecological studies are susceptible to a bias commonly referred to as the ecological fallacythe (often implied) assumption that correlations occurring at the populationlevel parallel correlations at the individual level 2. This assumption is sometimes true, but not always; indeed, correlations that exist when analysing populationlevel data may not exist or may even be reversed when analysing individuallevel data. Researchers must use individuallevel data to reliably discern the individuallevel effect of medical marijuana use, an individuallevel exposure.