Thanks Paige for your code. Reeza: What do you want me to explain? I apologize, being an international student, could not meet your standards in explaining things. However, let me try my best. I have my data under two separate columns: Column 1 (Month)               Column 2 (Customer ID) 201007                                   x1...xn .                                               x1...xn                                           . . . . . 201310                                   x1....xn I want my result in following format: Column 1 (Month)               Column 2 (New ID)               Column 3 (ID that exited) New ID Definition: ID that never appear in last 12 months but appear in current month. Exit ID Definition: ID that appeared 12 months before but never appeared for next 12 months (including current month). I hope I explained my problem. If you think I am still unable to explain things, please let me know and I am happy to try as much as I can. Thanks a lot for your help. ... View more