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Original Message (ID '367986') By yehuda:
It is more than a simple Case or Select (since then you will have to loop according to the values of the first element) 1. Transpose a (Thread also works) so you get a list of pairs, one from the first list and one form the second list 2. GroupBy to group these pairs according to their first element 3. Use Map at level 2 to remove the first element 4 Map the function Mean on the result so you get your results Mean /@ Map[Last, GatherBy[Transpose[a], First], {2}] result is {3,4} as expected HTH yehuda