I'm relating to this study: http://www.jneurosci.org/content/35/4/1505.short

The researchers found no differences in subcortical brain structures for daily Cannabis users vs. controls. However the controls were only abstinent for 60 (adults) or 90 (adolescents) days. IF Cannabis use was associated with changes in brain structures which take more than 60-90 days to normalize, these (presumably) insufficient abstinence periods could be the reason why no differences between user and controls could be found. Why did they not use a control group which had not used Cannabis at all?


2 Answers 2


The researchers used a questionnaire to identify daily users, that is, the experimental group contained persons who had used marijuana every day during the past 60 days.

The control group, on the other hand, consisted of persons who had not used marijuana at all during the past 60 days.

The researchers probably felt that 60 days (that is, three months or a quarter year) is a long enough period to establish habitual use and habitual non-use.

Of course it is possible that the 60-day-non-users did use marijuana once or twice every year, just as it is possible that the 60-day-daily-users did not use marijuana once or twice each year. But this should have no massive long term effects either way.

Of course it is possible that the non-users just stopped using 60 days ago, or that the users just started 60 days ago. But it is unlikely that this would be true for a significant proportion of participants, and for most participants it is very probable that the usage pattern over 3 months reflects long time usage.

But basically you are of course right. The worst case could be that the last-60-day-users had all just started 60 days ago, and that the last-60-day-non-users had all just stopped. The researchers might have asked this, but they do not report such a question, which, in my eyes, is a problem. But probability is in their favor.

  • $\begingroup$ You outran me with 5 min! Can you believe it :) +1 :) I totally agree. $\endgroup$
    – AliceD
    Feb 16, 2015 at 10:35
  • $\begingroup$ Thanks for your explanation! What IF some heavy Cannabis users stopped using the drug years ago? As long as there is no prove of full reversibility of Cannabis induced brain alterations I think it is premature to use a control group which did not use Cannabis for only 60 days (which is 2 months and 1/6 of a year btw. ;-)) I agree it would have been very interesting to know if some of these non-using controls had used Cannabis in the past (sporadically or excessively). However as you point out the study does not give any details about consumption patterns before the 60 (90) days. $\endgroup$ Feb 16, 2015 at 14:29
  • $\begingroup$ Ops, yes, 60 days = 2 months. My brain is fried from learning for my exams today. o_O Using made up numbers, try to calculate the mean of 20 users (2 x 10), then calculate the mean of 10 non-users plus 10 users (10 x 8 + 10 x 10). Still a difference (10 vs 9). And it is extremely unlikely that half the control group are long time daily ex-users. So it does not matter, because the false probands will average out! $\endgroup$
    – user3116
    Feb 16, 2015 at 15:01

Short answer
The control subjects were abstinent for at least 60 and 90 days, respectively.

The article you cite mentions the following in the Materials and Methods, and I quote:

From the remaining subjects, age, gender, and AUDIT scores were used to create a matched control group reporting no marijuana use in the past 60 d.

A similar procedure was used for the 90-day control group. I agree with you that this method, i.e., at least 60-90 days of abstinence, leaves plenty of room open for variance. Probably a large part of the identified controls have not smoked marijuana ever, others maybe once or twice, but who knows? I think you have identified a very good point of critique.

The article mentions two citations (p.1506, Materials and Methods, first paragraph: Filbey 2008, Claus 2011) with respect to this protocol, and I followed these as I was curious if they actually controlled for this shaky part of their method to generate a control group. The first reference is actually not in the reference list(!) and after a thorough search I dug it up. I couldn't find any details on the questionnaire, however, and the same holds for the other article. If it is important to you you may want to read these articles in full (I just scanned the section on the questionnaire) to search for clues. And contacting the authors is never a bad thing either. For now, I conclude you are right and the methods are shaky.

However, flawed as mentioned in your question title is a big word. As long as you take the methods used into account, you can place the results in perspective against other studies that, perhaps, used study participants that have never touched any THC-containing product in their lives.

  • 1
    $\begingroup$ Thanks for your explanation! I already contacted one of the authors concerning the issue about the 60days abstinent control group (Rachel Thayer). So far I did not receive an answer. I agree flawed is a strong word, but you might agree that the title of the study also makes strong claims. In the words of Carl Sagan : "Extraordinary claims require extraordinary evidence" The study fails to provide such evidence due the methodological weakness.. $\endgroup$ Feb 16, 2015 at 14:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.