There are a variety of reasons for this. I did not check all of the missing records, but here are some explanations: 1) m03 was just an MRI visit, so there was no clinical visit at which a diagnosis was made; 2) there are some ADNI2 participants who came in for the screening visit but then did not continue (no further visits), so their screening information is provided (but no baseline diagnosis, since the baseline visit did not happen); 3) some are phone visits only, so no clinical diagnosis is made. You can check the REGISTRY file to get more information on types of visits, and whether or not a participant had visits after the one that is missing a diagnosis.