Getting the average over a range of dates grouped by month for each account number.

[valerieraem]

I am trying to calculate a field using three different columns. I have a list of numbers that occur over the course of a few days in each month for each person in my database. I need to filter this down to having only one number from each month for each person. 

I thought finding the average or the mode of the number over each month for each person would be the best way, but I can't figure out how to do that. The number should be the same over each month for each person. If there is a better way please let me know. Any help is appreciated. 

This is what my table looks like currently