Statistics on the Prevalence of Drug Users Can Be Misleading

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.

Presentation2

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.

Comments

  1. Mike says

    My take on this is that manipulation of the data conflates two things: drug use and law breaking. Obviously, those with an investment in law enforcement as a drug use control mechanism prefer this mixing of goals. One demands the other, right?

    Problem is the definition of drug abuse is determined by frequency of law-breaking, rather than some sort of objective, medical definition of what drug abuse amounts to. That’s where our society went so badly wrong with marijuana. It’s stuck on the legal definition of marijuana as a Schedule One controlled substance, when there is virtually no medical data indicating physical harm and often questionable or exaggerated data about any psychological impact. In this case, one can throw all the crime statistics at the wall one wants and virtually none of it sticks as actual health impacts. And citizens aren’t stupid. The government can repeat this Schedule One mantra until the cows come home and it still won’t define reality.

    Drugs that do fall into the category of potentially dangerous are somewhat different. Obviously, cocaine can kill you. Just as obviously, people use or abuse cocaine regularly and suffer no fatal consequences — kind of like acetaminophen… ;)

    Now, if you compare rates of use (or abuse, if you must) among different drugs between racial categories, it also involves all sorts of issues that have little to nothing to do with actual harm, but do reflect the legal pressures of enforcement as they are skewed by racial disparities that exist in our society. For instance, selling cocaine in rocks as crack was a response to law enforcement pressure. An easy to conceal and preserve form of cocaine, it is packaged to facilitate a quick high and drive the user back to repeat. So one can easily see multiple sales as a statistic being very skewed versus the same total amount of powder cocaine bought all at once by middle-class, often white users. Some may argue the same amount of cocaine in crack is far more dangerous than in powder form — a wholly facetious argument that the US Congress bought to the tune of a 100-to-1 disparity — which is for all intents and purposes a racial disparity that has more to do with how law enforcement pressure has skewed its practices to gain the greatest number of years behind bars toward African-Americans. Are crack users actually hurting themselves more than powder cocaine users? Perhaps, but it’s not something that is reflected in these sorts of statistics.

    I think it would be very useful if those concerned about reducing drug abuse start developing metrics that do not rely on law enforcement statistics as measures of progress. One thing they uniformly fail to acknowledge is that if we measure abuse by arrest statistics, this has little relationship to the harm that is done by drug abuse — and is certainly no measurement of whether law enforcement does anything positive in relation to drug use. After all, if arrest statistics are a measure of drug abuse, we can easily end drug abuse by simply no longer arresting people for it. Poof! The problem is solved. I know someone will immediately say, “How ridiculous!” But it’s equally ridiculous to use law enforcement-oriented metrics to assess what is fundamentally a health problem, even where it’s not a medical issue at all.

  2. Ed Whitney says

    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).

    • Jon Caulkins says

      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.
      Jon

      • Ed Whitney says

        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.

  3. Ralph says

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

    • Jon Caulkins says

      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).

      • Ed Whitney says

        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

  4. Bruce Ross says

    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?

  5. ProfNickD says

    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?

    • Jon says

      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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