How Do PROs Track Media Use on Terrestrial Radio?

Usually, a PRO will apply to types of surveys across media: Census and Sample.

A Census Station will pay a large enough licensing fee to the PRO which makes it economically feasible to distribute royalties for every airing/broadcast/performance.

Sample Surveys however are applied to stations that pay a rather low licensing fee, in comparison, and therefore, it is not economically feasible to try and process every performance which will amount to a rather low payment. Therefore, a small selection (each PRO is different) is taken of performances from that licensee/music user and processed accordingly.