Contrary to what we expected, geographic location of the studies, which is a broad index of different cultures and contexts, was not associated with significant variability in prevalence estimates of CD or ODD. The variability in prevalence was mostly related to methodological differences across studies. Both the requirement of impairment for diagnosis and variability in diagnostic criteria were significantly associated with the heterogeneity of prevalence estimates for CD. For ODD, only age was significantly associated with heterogeneity. For CD, requirement of impairment for diagnosis was associated with a lower prevalence estimate in comparison to no requirement, as well as DSM-IV in comparison to DSM-III-R. For ODD, children more than 12 years had lower rates than children with less than 12 years.
