Question

At a police checkpoint for drunk drivers, some cars are stopped at random; many others are not stopped. The cars that are stopped may be considered a _____ of all the cars that pass by the checkpoint.

Answer 1

sample

or spot check

Explanation:

there are quite a number of different terms that can be used depending on the specific scientific field you are working in.

but "sample" should be the most common one for a random selection of a subset of the whole set.