Hi Monica - you need to use what we call $(dollar) expansion - it enables the Set Analysis function to use another function in the comparison.
I can work out a sample for you - but in the mean-time -I discuss this in part 2 of this Set Analysis Introduction as well:
Part 2: Set Analysis - Part 2 - Cool Stuff You Can Do (video) at the 11:45 mark
Let me see if I can quickly get you a sample.
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BTW - here is an example in the community
Please note that you can only use field names on the left of the equal sign in a set expression, no functions.
Let me know how you do