Use the sum for CTR if it is not aggregated in the calculated field like below
IF (ZN(SUM([CTR])) - LOOKUP(ZN(SUM([CTR])), -1)) / ABS(LOOKUP(ZN(SUM([CTR])), -1)) > 0 THEN 'Up'
Change the compute using because you are using the table calculation so make sure the your addressing and partitioning is correct for the LOOKUP function to calculate
Hope this helps
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Thanks for the reply!
CTR is a calculation I created that takes (sum of clicks)/(sum of impressions).
When I do SUM or AVG in the Up or down calculation, I get an error that says CTR is already an aggregate.
I know the % dif calculation is correct because I am getting the right numbers.
What do you mean by addressing and partitioning?