Statistics on the Prevalence of Drug Users Can Be Misleading

A new paper shows how drug use prevalence statistics can skew perceptions of drug policy

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 proportion 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 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.

Author: Keith Humphreys

Keith Humphreys is the Esther Ting Memorial Professor of Psychiatry at Stanford University and an Honorary Professor of Psychiatry at Kings College London. His research, teaching and writing have focused on addictive disorders, self-help organizations (e.g., breast cancer support groups, Alcoholics Anonymous), evaluation research methods, and public policy related to health care, mental illness, veterans, drugs, crime and correctional systems. Professor Humphreys' over 300 scholarly articles, monographs and books have been cited over thirteen thousand times by scientific colleagues. He is a regular contributor to Washington Post and has also written for the New York Times, Wall Street Journal, Washington Monthly, San Francisco Chronicle, The Guardian (UK), The Telegraph (UK), Times Higher Education (UK), Crossbow (UK) and other media outlets.

12 thoughts on “Statistics on the Prevalence of Drug Users Can Be Misleading”

  1. It is an interesting paper simply for its exploration of the consequences of using different metrics of cannabis use. I had one minor issue and a couple of major issues with the reporting, however.

    Minor issue: Table 2, the educational level among adults, proportion of past year users (upper left column) adds up to 110% instead of 100% as the other columns in the table do. Perhaps the college graduate cell should be 10% lower but the authors would know how to make the correction.

    First major issue: it is often hard to interpret percentages without having numerators and denominators. Table 1 had five different measures of cannabis participation. There would have to be five different denominators (past-month users will be included in past-year users, but we are not told what n is for either measure; n for past-year will be larger than for past-month, but how much larger?). Past-month purchases is likely to have a larger n than past-month purchasers, since many purchasers will make more than one purchase, but Table 1 does not report n for either measure.

    Second major issue: The data combine medical and recreational use of cannabis. It is reasonable to speculate that the legalization of medical marijuana in several states over the past decade accounts for some of the upward trends seen in Figure 2 and in Figure 5. In addition, if White Non-Hispanic users are disproportionately represented in medical cannabis consumption (a reasonable speculation since this is a proxy for access to health care), then their risk of arrest would be lower than for other groups. Age group is also likely to be associated with medical cannabis consumption since many conditions (such as glaucoma) are rare in younger groups.

    This could be a pretty good paper for a journal club somewhere, with much useful information and many points of discussion as to the gaps in the data (perhaps enough to give someone a thesis topic to work on).

    1. Ed:
      You are correct about Table 2. The 31% for college grads should be 22%. Thanks for catching that! (22% makes the column sum to 101%, but that’s a rounding issue.)
      Yes, the n’s are all different by measure: 29 M past-year users, 18M past-month users, 247M days-of-use, 9M past-month purchasers, and 60M past-month purchases.
      I don’t think the growth in medical MJ changes the qualitative results in the paper overall — by which I mean the principal finding that measure of use matters and when possible prob better to focus on days-of-use not PY users. We see broadly similar patterns in older HH surveys. But Keith’s comments focused on the buys vs. other measures of use for AA. That particular result may change over time; I just don’t recall for that outcome and don’t have time to re-run with a different survey now.

      1. The medical cannabis does not affect the main points of the study, but it does have some relevance to the part of the table that Keith had reproduced dealing with arrests for drug offenses. I do not know the racial/ethnic distribution of medical marijuana purchasers, but if a medical marijuana card is associated with race (whites more likely to have these cards), then this could be a confounder for the data in the second column of that table. Arrests of purchasers are rare at dispensaries but more common in street corner deals, I would suppose. The conclusions about the relevance of consumption metrics are not affected.

  2. Lots of people use drugs but never make a buy: folks who grow.
    And I also agree that consumption data are needed.

    1. The surveys ask about growing; very few people report that.

      The great bulk of people who don’t buy report receiving MJ most recently for free (think sharing among friends).

      1. Some users may pay for their own cannabis but may send someone to make purchases, especially in the black market. If illicit dealers are cautious about their customers and do not want to sell to people they do not know, then such arrangements could be common in the street market. The regular users who send money with their designated purchasers are not receiving it for free. Is this a common arrangement? The rules at licensed dispensaries are, of course, different where designated purchasers are concerned

  3. Buying is surely a good indicator of behavior likely to draw notice of the police. Is is it a good indicator of dangerously abusive/addictive use? Does any such indicator exist? Does anyone survey for it? Or is the sample size to small to capture?

  4. I’ve seen data like this before and it is often inferred but not really explained that Blacks must be buying in far smaller quantities than White or Hispanics At least, this would seem that would be the only explanation for close to double the drug transactions as a function of drug use.

    But is that true?

    1. Because of the nature of the questions, a full answer is more than a quick cross-tab, but basically yes.
      First question those who purchase hit is did they buy joints or loose, and almost everyone says loose. (A few more AA than W say joints, but it’s very small in either event.)
      Next Q asks what units they’d like to use to describe their most recent (loose) purchase: grams, ounces, or pounds. Very few answer in pounds, so it’s mostly between grams and ounces.
      For NH Whites, 26.3% report their purchase in ounces, vs. 17.8 for H and 17.3 for NH Blacks.
      Among those answering in grams, there are 3 responses at the next level: 1-5, 5-10, and 10+. 37% of NH W answer 5-10 or 10+ vs. 26.5 for AA.
      So in short, “yes” to some extent.

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