ALSPAC is subject to attrition, which is higher among the socially disadvantaged and increases with participant age. This may have implications for external validity, as we have discussed previously 10. It is also possible that we underestimated the prevalence of problem cannabis use as this outcome was associated with earlier exposures that, in general, were more common among the disadvantaged. Multiple imputation of missing data made little difference to either our estimates of the prevalence of cannabis use or problem use at age 16 or to our estimates of the effect of earlier exposures on these outcomes (full data for comparison available as a web supplement). There are few available UK data with which to compare our estimates. In the 2010/11 sweep of the British Crime Survey, 22% of male compared to 13% of female respondents aged 16–24 years reported any use of cannabis in the past year 32. For daily or almost daily use the equivalent proportions were 13 and 8% in this age group. In a recent large UK school-based study, 22% of 15 year-old boys and 20% of 15-year-old girls reported any use of cannabis in the previous year 33.
