In general, Research Group in the IDA refers to the screening diagnosis (in the ARM table for ADNI-1/GO/2; Diagnosis in the Diagnostic Summary file for the screening visit). There may be some instances where this doesn't quite match, which could be due to updates made to the data from the site, which would not change the Research Group designation in the IDA. The Clinical Core generally recommends using the diagnosis at the baseline visit (not screening visit) as the initial diagnosis, since this diagnosis is based on the full information rather than just what is collected at screening. The screening diagnosis, especially in earlier phases of ADNI, determined the visitation schedule.