In their recent paper, Moffitt et al.(2009) claim that lifetime prevalence estimates based on retrospective surveys may be half as large as they would be if they were computed as cumulative incidence on the basis of prospective data. They also infer that the persons who are counted as having lifetime disorder in the retrospective surveys over-represent those who have current disorder. The authors base these claims on a string of inferences that they believe allows them to compare lifetime prevalence estimates in the Dunedin cohort to retrospectively ascertained lifetime prevalence estimates from three different large-scale crosssectional community surveys. There are several features of the comparison that could raise concerns about the conclusions. For example, the retrospective lifetime estimates are restricted to persons aged 32 and younger and in the Dunedin cohort the cumulative estimates are based on a combination of assessments of 12-month prevalences at only four time points, corresponding to ages 18, 21, 26 and 32. In addition, the Dunedin group used DSMIII-R at two time points and DSM-IV at two others, and unlike the cross-sectional surveys, the Dunedin investigators used trained clinical (rather than lay) interviewers to administer the Diagnostic Interview Schedule (DIS). Moreover, the Dunedin study diagnoses used a rating of impairment and also reported symptoms. Finally, the study populations in two of the comparisons were qualitatively different: Dunedin is a single location in New Zealand and the National Comorbidity Survey (NCS) and the NCS Replication (NCS-R) are representative samples of the USA.

Although these features could undermine the comparisons of lifetime estimates from Dunedin to estimates from the three other studies, they would be expected to have the same effect on 12-month prevalence estimates as on lifetime prevalence estimates. Moffitt and her colleagues argue that their 12-month results are in fact quite close to the other studies for any anxiety disorder, and also for panic, specific phobia, social phobia and generalized anxiety. We accept the argument for panic and specific phobia and for the comparison between Dunedin and NCS-R for any anxiety. In other instances, however, the amount of overlap of the confidence intervals is consistent with reliable differences (Cumming & Finch, 2005); factors such as the lower refusal rate might have contributed to higher 12-month prevalence estimates in Dunedin.