Aggr Nested in Set Analysis

[#1]

A simple example but I am stuck ...

Previous Year

=Sum({$<ActivityStartYear={$(=Only(ActivityStartYear)-1)}>}aggr(ApprovedPC, ActivityID)) is not returning any results

Selected Year

=Sum({$<ActivityStartYear={$(=Only(ActivityStartYear))}>}aggr(ApprovedPC, ActivityID)) ... is ... the difference being the -1

Basically I am trying to do a simple year on year table but need to aggregate the results by ActivityID before summing the value.

Could anyone explain why it fails on previous year but selected year is OK?

Thanks

Lee

[#2]

Martina/Karl ... brilliant! That has cracked it, thank you so much for your help on this ...