It's true that I don't like the result of this study. My experience is that plenty of healthy, smart, motivated, effective people consume cannabis quite regularly. I have been smoking marijuana quite regularly since I was about 19 years old (which is 13 years ago now) and of course I don't like to think of myself as cognitively impaired.

However, I think that studies like this are particularly vulnerable to this critique of methodology. Any time you assess the performance of a large group and then divide that group into subgroups, you are going to find trends of varying levels of statistical significance.

It is incredibly easy to do. http://www.tylervigen.com/

That is why, in every scope-and-methods class in every college across the country, the various methods for ascertaining causality are demonstrated and taught.

Reading only the linked article and not the study itself, I so far have the impression that none of these were seriously considered - presumably on the basis of ethical treatment of humans subjects. You can't just take a group of people with a particular set of desirable control features (perhaps, in this case, a typically shaped hippocampus) and force-feed them cannabis smoke for three years.

On the other hand, it's just as easy to read this study as meaning that people who performed poorly on the relatively specific requirements of this study seemed to prefer to smoke marijuana. If this study were about serious science, the conclusion might be the utterly uninteresting sky-is-blue headline, "absent-minded people likely to smoke weed."

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