If you do by make:, and make indeed uniquely identifies a car, then each *by* group will have exactly one car in it. That is in fact the case:

bysort make: l make

If there were two cars with the same value of make they'd be in the same *by* group. Thus in that group (and only that group, assuming no other duplicates) there would be an observation two--we'll take advantage of that fact in Working with Groups.

Last Revised: 11/19/2009