Colorado Boulder study about brain morphometric measures of Cannabis users methodologically weak? 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?
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.