I have the following data:

date days
06/06/08 12
11/06/08 8
13/06/08 21
16/06/08 5
17/06/08 18

I need to show this as a pivot table

month ave. -top20% bottom80% total

May xxx xxx xxxx
Jun 21 10.75 12.8

There are 5 entries so 21 is excluded from the bottom 80% as it represents
the highest 20% of cases.

I am using Excel 2007 and it looks as if I should be able to do it. I can
do it without any grouping but i get wrong results when i group.