In Reply We agree with Finney et al that large, prospective studies of the effects of medical cannabis in individuals with chronic pain are warranted. However, we believe that results from studies of state-level data can also be informative.
Robinson1 identified a fundamental concept, that associations between variables at the individual level may be different than associations between variables at the group level. However, group-level studies can be of great value when the unit of inference is the group (in the case of our report, the state) and individual-level data are unavailable.2 Furthermore, we are not simply comparing states with and without the policy at a single point of time, an inferential method often leading to flawed ecological correlations as Robinson1 illustrated. Rather, we are comparing changes before and after a state-level policy change relative to states where there was no policy change. This over-time variation allows us to statistically adjust for many of the idiosyncratic state-level differences that might otherwise lead to biased conclusions.3
Bachhuber MA, Saloner B, Barry CL. What Ecologic Analyses Cannot Tell Us About Medical Marijuana Legalization and Opioid Pain Medication Mortality—Reply. JAMA Intern Med. 2015;175(4):656-657. doi:10.1001/jamainternmed.2014.8027