Data from each study was independently extracted by two reviewers and then finalised by consensus. We extracted data on current and lifetime prevalence rates for all reported comorbid diagnoses. If a study did not state explicitly the examination of lifetime vs. current comorbidity, we subsumed reported rates under lifetime comorbidity. Other data extracted from each study included sample size, mean age at assessment (AAA), mean age of onset (AOO) of OCD, gender distribution (percentage of males), OCD illness severity rated on the Yale-Brown obsessive compulsive scale (Y-BOCS) (22, 23) or child Y-BOCS (CY-BOCS), respectively (38), country in which the study was carried out, time period of data collection, classification system used, diagnostic instrument used, study design, and recruitment source of participants [e.g. clinic-based (from treatment-seeking individuals presenting to the clinic) vs. community-based (from the general population), i.e., community based studies included all referrals from the community while the clinics only serve a selected proportion of all patients in the community]. Where reported, comorbidity data was extracted for both broad categories (e.g., mood disorders, anxiety disorders), as well as individual diagnoses
