Summarizing summaries

Hi, I have a set of data which is grouped and I have a Formula Field in the group footer that calculates the precision (i.e. StdDev/Mean *100) of the data for each group. For 4 groups this might give me the values 91, 93, 96, 89. How do I calculate the precision of those 4 values and place the result in the report footer, i.e. I want the precision of the precision. I've tried to use a Report Field within a Summary Function such as Average but I get the error message 'This field cannot be summarized' so I can't even calculate the average of those 4 values. Is what I'd like to do possible?

Edited by: Colin Winton on Sep 8, 2010 7:21 PM