Mark all observations that has one variable in common

[Cathrine Christiansen]

Hello, I'm quite new to stata. I have searched in this forum, but can not find the answer I need, though it is probably a very simple solution.

I have a dataset with 5 mill. observations of different happenings to approx. 2 mill. individuals. So for each individual ("PatientID") there are several observations. I want to keep only those individuals who have one variable ("acne-pat") in common. So for each individual I have "acne_pat"==1 in one of the individuals observation. How can I keep all the observations for thees individuals, and drop the rest? The result I want is the variable "Var2" in the example dataset.

* Example generated by -dataex-. For    more    info,    type    help    dataex
clear
input byte(PatientID acne_pat Var2)
1 1 1
1 . 1
1 . 1
2 . 0
2 . 0
3 1 1
3 . 1
3 . 1
4 1 1
4 . 1
5 . 0
5 . 0
5 . 0
end

Thank you,
Cathrine

[Fei Wang]

bys PatientID (acne_pat): gen Var = !mi(acne_pat[1])

[Cathrine Christiansen]

Thank you so much!

[Nick Cox]

See also FAQ https://www.stata.com/support/faqs/d...ble-recording/