I’m having a problem with an error message “this field cannot be summarized”.

I am attempting to count customers that have one or more orders with specific items during a set time period.

The data looks like:

Id            order date         types  

1              4/1/09                   1,2,,,,,8,,

1              4/3/09                   1,,,4,,,8,,

1              4/9/09                   1,2,,,5,,,,

2              4/9/09                   1,,,,,,,,,,,

2              4/25/09                ,,,,,,,,8,,,

5              4/18/09                1,,,4,,,,,,

I want to count all customers with one or more occurrences of items 2 or 8.

The above data should return a count of 2.

So far I have:

1) grouped on Id

2) created a formula to return 1 or 0 for each detail line occurrence

3) summed the occurrence values in the group footer

4) created a formula to return 1 for any group sum >0

I then attempted to sum the values from step 4) in the report footer. This is when I got the error message “This field cannot be summarized”

I’m still very new to Crystal Reports.  There may be a much easier way to get at this information.  As the nature of this report will be a common result we are looking for, I’d also like to learn what determines whether a field can be summarized. I looked in my reference books and didn’t have any luck.  Any help would be greatly appreciated. Thanks