Hi!

I'm working with data collected in 2002 (round 1), 2007 (round 2) and 2017 (round 3). I want to drop all the observations that participated in only one ofte the rounds, and only keep observations where the participant participated in two or three rounds.

I have a variable that indicates the reference number (ref_nr) of the participant. So in other words, if a person participated in both round 2 and 3, he or she will have the same reference number in both rounds.

I'm therefore wondering how I should write the command? I know the following commands doesn't work, but I'll write them as examples for you to understand what I'm after:

drop if ref_nr appear once

Or on the other side:
keep if ref_nr appear more than once

Thanks in advance!