Difference between two grouped values

Question

Hi, I need your help,

I have developed a comparative report like this:

                    01:00     02:00
Desv              10         23
15/01/2009     20         22
16/01/2009     30         45

I have a dataset with values from two different dates every 15 minutes. I have a matrix which is grouped by row Date and grouped by column Hour. I would like to show values every hour (making the average of the values) and I would like to calculate the difference between agruped values every hour. However, i cannot make first(avg(fields!info.value))-last(avg(fields!info.value)) How can i do this? please help me!