Marijuana legalization advocates often rest their case on the large absolute number of marijuana simple possession arrests made each year in the U.S. (e.g., over 650,000 in 2011). But number of arrests is just a numerator. If we want to understand the intensity of marijuana possession enforcement in the US, we also need to know the denominator, namely how often Americans consume marijuana.

Americans’ aggregate days of marijuana use for 2002-2011 can be derived from the public use dataset of the National Survey on Drug Use and Health (NSDUH)*. I combined that information on the denominator with FBI Uniform Crime Reports data on annual marijuana possession arrests over the same period.

Enforcement intensity was fairly consistent in the seven years of data from the George W. Bush Administration, with an average of .30 simple possession arrests for every 1,000 days of marijuana use. The absolute number of marijuana possession arrests over the GWB years went up from 614,000 in 2002 to 754,000 in 2008, which at first blush suggests sharply increasing enforcement intensity. But the number of possession arrests per 1,000 days of use in 2008 (.296) was virtually identical to that of earlier years (e.g., .294 in 2003) because frequency of marijuana use increased by roughly the same amount as did arrests. This is a concrete example of how interpreting numerators without denominators can be misleading.

In addition to noting the consistency of marijuana enforcement intensity during the GWB era, it is worth pondering how low a risk of arrest a rate of .30 per 1,000 use days reflects: If you smoked marijuana once per week, you would expect to be arrested for simple possession once every 64 years.

Matters become even more fascinating under the Obama Administration. As disappointed marijuana legalization advocates complained at the time, the first year of President Obama’s Administration saw an almost identical number of simple possession arrests (758,600) as did the last year of the George W. Bush Administration (754,200). But this represented a significant drop in enforcement intensity because Americans’ marijuana use increased by 10.6% that same year, from 2.55 Billion to 2.82 Billion aggregate days. As a result, in the first year of the Obama Administration, enforcement intensity was already lower than at any point in the GWB data.

During the next two years of the Obama Administration, Americans’ aggregate days of marijuana use continued to rise (This was driven somewhat by an increase in the total number of users, but even moreso by an increase in the size of the subpopulation of users who use every or nearly every day). Meanwhile the number of simple possession arrests actually fell (to 751,000 in 2010 and then 663,000 in 2011). The combination of these two trends produced a steep decrease in the intensity of marijuana enforcement.

The chart below summarizes the data. The baseline “GWB average” is the .30 average rate of marijuana possession arrests per 1,000 days of marijuana use from 2002-2008. For the last available year of Obama era data, 2011, enforcement intensity is down a remarkable 29.7% relative to the standard under the prior administration.


Chart notes: “GWB Average” is based on 2002-2008 because 2001 NSDUH data on drug use are not comparable to subsequent years due to survey design changes. Uniform Crime Report data is on number of arrests, potentially including multiple arrests of the same individual.

*I am very grateful to Dr. Beau Kilmer for helping me understand these data.

[Cross-posted at The Reality-based Community]

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Keith Humphreys

Keith Humphreys is a Professor of Psychiatry at Stanford University and served as Senior Policy Advisor in the White House Office of National Drug Control Policy in the Obama Administration. @KeithNHumphreys