Three important methodological weaknesses of the ENIGMA‐OCD concept should be mentioned. First, the data collection was done in the past, at a time when there was no plan to use the data in worldwide collaboration. Therefore, there has been no harmonization of data acquisition, either with respect to imaging data or with respect to inclusion criteria and clinical data. Various MRI machines (with different field strengths) and various scan sequences have been used with inevitable variations in data quality. In addition, the level of detail on the clinical phenotypes (disease severity, comorbidity, disease course, treatment history, etc.) is limited, so disallowing detailed analyses of symptom profiles or cross‐disorder characteristics. Second, the data collection has been mostly cross‐sectional, limiting our ability to answer questions related to disease course, effects of medication, and cause versus consequence of disease. Longitudinal data is highly warranted, and with the increase in pre‐/post‐treatment imaging studies, we expect that more longitudinal data will soon become available. Family and particularly twin studies could go a long way to disentangle vulnerability from disease chronicity and medication. Third, only some of the imaging samples also have genotyping data, presenting a challenge, since for imaging‐genetic analyses large samples are needed.
