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Message: [QUOTE="Francisco, post: 3859057"] Mike, Many thanks for you time. It is my fault I do not expressed myself very well. I have 4 companies, each one has a different staff number: List1: CompID StaffID 1001 67 1001 2 1001 89 1001 22 1001 98 1002 70 1002 65 1002 9 1003 29 1003 97 1004 63 Company 1 (1001), has 5 people, Company 2 (1002) has 3 people, .... and so on. So I have created a summary by Number of staff, the list look like this: List2 CompID NoStaff(count StaffID) 1001 5 1002 3 1002 2 1002 1 The STDEV on the summary (list2) is STDEV(B2:B5)=1.707885 The problem I have is I can not calculate the STDEV based on List2, I need to work on List1 for another reasons. I asume I need to have an array formula to group then number of staff by Company on the fly, and then calculate STDEV. The question is, how can I do this on a single formula? The final result must be 1.707885. The formula will look like ={STDEV(No Staff group by CompanyID)} [/QUOTE]