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I have a panel data set which contains several series. I need to sum up the values for some cross sections (across identifiers, not time) depending on some specific conditions. I thought of something like this:
However, the values of test appear to be summed up across cross sections and over time. Is it necessary to write a loop which repeats the above procedure for each period separately or is there a simple command to do that?

Use @sumsby(series_a, @crossid) (or replace @crossid with the name of your cross-section identifier series).