Abstract
Evaluations of the impact of medical and recreational marijuana laws rely on quasi- or natural experiments in which researchers exploit changes in the law and attempt to determine the impact of these changes on outcomes. This chapter reviews three key issues of causal inference in observational studies with respect to estimating of impact of medical or recreational laws on marijuana use—intervention definition, outcome measurement, and random assignment of study participants. We show that studies tend to use the same statistical approach (differences-in-differences) and yet find differential impacts of medical marijuana laws on adult use in particular. We demonstrate that these seemingly conflicting findings may be due to different years of analysis, ages of the study sample in each year, and assignment of jurisdictions to the control group versus treatment group.
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- 1.
More specifically, all null hypotheses (at least in their two-tailed forms) are false (Cohen 1988); therefore, a change in policy must have an effect—the question is the size and direction of the effect.
- 2.
Although the RCT is considered the ‘gold standard’ for establishing causation, it is interesting to note that the first RCT in medicine was published as recently as 1948.
- 3.
Arizona voters also passed a set of laws permitting the use of marijuana for medicinal purposes in 1996; however, the statute included language allowing physicians to prescribe MM (Pacula et al. 2014).
- 4.
For reports for WA, see http://liq.wa.gov/marijuana/botec_reports.
- 5.
“The standard deviation for the treatment group is unaffected by propensity score weighting and allows for comparison pre- and post-weighting.”
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The authors would like to acknowledge funding from the National Institute of Drug Abuse, grants #R01DA032693 and #R01DA032693-03S1.
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Hunt, P.E., Miles, J. (2015). The Impact of Legalizing and Regulating Weed: Issues with Study Design and Emerging Findings in the USA. In: Nielsen, S., Bruno, R., Schenk, S. (eds) Non-medical and illicit use of psychoactive drugs. Current Topics in Behavioral Neurosciences, vol 34. Springer, Cham. https://doi.org/10.1007/7854_2015_423
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DOI: https://doi.org/10.1007/7854_2015_423
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