st: control selection - looping over observations not possible - alternatives

[raoul reulen <r.c.reulen@gmail.com>]

Dear Statalisters

I need to select up to 20 controls for each of 10,000 subjects from a
dataset of around half-a-million subjects. The controls need to
satisfy certain criteria (e.g., same age). How can I do this without
having to loop over observations? Thanks.

Raoul
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