Crystal Reports Groupings

Message

   I am working in Crystal 10... I have a formula that returns a number (1 through 7) to determine a group.  The formula works correctly, but then I want to add a group to the report based off of the formula.  I click on INSERT > GROUP and in the drop-down box, my formula is NOT listed... WHY??

The formula is this...

iif ( sum(0,1">{@IsEmb})>0,1,
        iif({vSJFS_PackListData.CSR}="INT",2,
        iif({vSJFS_PackListData.CSR}="CAN",3,
        iif(SUM(0">{@IsCR})>0 AND SUM(0,4">{@IsSJ})>0,4,
        iif(SUM(0,5">{@IsSJ})>0,5,
        iif(SUM(0,6,7">{@IsCR})>0,6,7)))))
);

Is it because the formula is too complex? Any help would be wonderful!!