August 13, 2007

From Paul Wakfer:

Re: Pot could boost psychosis risk later in life, study finds (July 27): it is stated:

In the new re­search, The­re­sa Moore of the Un­ivers­ity of Bris­tol, U.K., and Stan­ley Za­m­mit of Car­diff Un­ivers­ity, Wales, an­a­lyzed 35 past stud­ies on ma­ri­jua­na up to last year. They found that peo­ple who had used can­na­bis were 41 per­cent more likely than those who had nev­er used it to have any form of psy­cho­sis. The risk rose with dos­age, the re­search­ers added. They cal­cu­lat­ed that about 14 per­cent of psy­chot­ic episodes in young adults in the UK would not oc­cur if can­na­bis weren’t con­sumed.”

The last sentence in this report (particularly the phrase “would not occur if cannabis weren’t consumed”) was *not* a conclusion of the authors of the report and would not be a conclusion of anyone who understands scientific method.

The good evidence found for correlation between the use of cannabis and later psychotic episodes, does *not* imply the causation of such episodes by the use of cannabis. Because of that fact of scientific epistemology, it is even incorrect for the researchers to refer to use of cannabis as a “risk” for psychotic episodes. The increased likelihood of psychotic episodes among those using cannabis could just as easily be because those more likely to later have psychotic episodes are also more likely to choose to use cannabis.

One way to resolve this question would be to use psychological personality index testing on a group of cannabis users and a group of matched non-cannabis users to see whether or not those using cannabis have a personality that is more likely to lead to psychosis. If this has been done, there is certainly no mention of it in the Lancet abstract (I do not have access to the full paper).

(Editor’s note: We disagree that we misquoted or misrepresented anything in the study. Contrary to present author’s assertion, the Lancet paper indeed states: “...we can estimate that about 14% of psychotic outcomes in young adults currently in the UK would not occur if cannabis were not consumed.” However, the study authors also acknowledge that this is an estimate only, and possibly wrong. A copy of the full text is here.)


Anonymous Paul Wakfer said...

I apologize for suggesting that World Science editors might have misquoted the study authors. Although I had not seen the full paper (as I stated in my comment), my statement was based on the presumption that the authors would correctly understand that correlation and cause are not equivalent. Now I find that the situation is far worse than I imagined. Since there is no causal implication whatever from a pure correlation, it is impossible for there to be any valid estimate of causal effect. Therefore, when these authors state that they have "estimated" a 14% increase in psychotic events as a result of using cannabis, they are now mistaking the meaning of estimate with that of completely unsubstantiated opinion.

August 13, 2007 5:53 PM  

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