medication‐free subjects with OCD to date. Specifically, we will recruit 250 medication‐free adults with OCD, 100 unaffected adult siblings of individuals with OCD, and 250 healthy control subjects. All will receive clinical evaluation, neurocognitive assessment, and MRI, focusing on morphometry (T1‐weighted MRI), structural connectivity (DTI), and functional connectivity (rs fMRI). To ensure harmonized data collection, we have developed detailed protocols to ensure cross‐site reliability on clinical measures, standardized delivery of neurocognitive tasks, and optimal MRI data acquisition for data pooling (see Simpson et al., 2020for details). Following best‐practices used in large‐scale imaging consortia, the MRI quality procedures include: Harmonization of scan sequences to optimize scan quality across scanners; both human and physical phantom studies at study start, with repeat phantom studies and preprocessing of all scans in real‐time to ensure maintenance of scan quality over the course of study recruitment. Other ENIGMA‐OCD sites that start new data collection might consider using similar methods, to increase the number of samples with higher quality harmonized data.
