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November 15, 2013 4:32 AM Statistics on the Prevalence of Drug Users Can Be Misleading

By Keith Humphreys

Drug policy development and analysis often focuses on the prevalence of users, most commonly the proportion of the population that has used one drug or another in the past 30 days or past year. These are the data that are typically being cited when people say things like “Drug use is down/up by 10%”. But the prevalence of people who have used a drug in the past 30 days or year is a crude statistic that papers over extraordinarily diversity in these users’ drug consumption and consequences thereof.

For example, if everyone who uses cocaine once a month starts using it once every five weeks instead while everyone who uses it 2-5 days a month starts using it every single day, a politician could point to the changed prevalence of past 30-day cocaine users and claim a big drop. But the total amount of cocaine being consumed would have risen sharply, as would the number of people who were getting addicted or experiencing other damage from cocaine use!

Among U.S. policymakers, The Obama Administration is I believe the only one in history to embrace this reality by setting no goal for the prevalence of drug users within the adult population. Rather, the goals of the drug strategy for people over the age of 25 are to reduce the number of chronic heavy users of hard drugs and the prevalence of drug-related morbidity and mortality.

In the research world, the wizards of drug policy at RAND Corporation have taken on the issue of simple prevalence measures in a new journal article that is available for free here. In a series of helpful examples, they show how reliance on these measures can skew perceptions of a range of drug policy issues. The example below comes from their paper.

The left two columns of the table tell a familiar story: African-Americans make up a much higher rate of drug offense arrestees than they do past-year drug users. From this one might draw a range of conclusions, for example that decriminalizing possession would reduce the African-American arrest rate more than the White arrest rate, or, that interventions to reduce racial discrimination in policing (e.g., stop and frisks that lead to drug arrests) would particularly lower the African-American arrest rate. However, these conclusions are based on the assumption that all past-year drug users are the same, which the right column of the chart shows is false.

With the same crudely-defined prevalence category (past year drug use, yes or no) exists substantial diversity on another dimension: Frequency of making drug purchases. Whites account for a smaller proportion of purchases than their proportion of past-year users would lead one to expect, whereas as African-Americans show the reverse pattern. There are many reasons this could be so — Whites buying drugs in bulk more often, Whites more commonly sharing drugs in a social network with only one purchaser, dealers being more available in some neighborhoods than others — but whatever the mechanism, the data in the third column suggest new interpretations that were hidden when the measure was simply past-year use. If arrests are mainly about purchases for example (and they do track them remarkably closely), decriminalizing simple drug possession might not affect African-Americans disproportionately after all.

Does this mean that prevalence of users statistics are inherently deceptive? Only if they are used to mean something that they don’t. But even when they are interpreted correctly, it still leaves observers in the dark about much of the reality of drug use in a society. That’s why we need other measures such as volume of consumption, purchasing pattern and prevalence of addiction.

[Cross-posted at The Reality-Based Community]

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Keith Humphreys is a professor of psychiatry and behavioral medicine at the Stanford University School of Medicine.

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