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