Dear all,   Sorry to bother you for a question for selecting of first observation based on sorting by several variables. The dataset was sorted by ID, diagnosis and date of diagnosis. I would like to select the first observation with the first diagnosed date but the same diagnosis.   I tried to use "first.ID and first.date" or "first.ID and first.date and first.diagnosis"but it seems not correct. I attached the test dataset. The records that I would like to select were highlighted in yellow.    Many thanks in advance!   Best Sisi ... View more