This can be done using scripts. It can a bit daunting if you haven't used scripts before and it can try your patience making sure everything is setup correctly, but it can be done.
The idea is that on form load you declare global variables for the different categories. Then in the detail, on the category field or the number field (that you are trying to sum) you add a script that contains the conditions such IF category = "cat1" THEN cat1sum = cat1sum + numberField. After adding the conditions for all the categories you would add a label for each category in the footer that has a script that enters the sum in to the label.
Hope that helps!
Matt Raiser
Founder,
www.techinthefield.com
matt@techinthefield.com
219-221-9500
~ Method user / partner since 2008
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