system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a radio station determine the popularity of their shows?

Obviously they'd need stats to know whether a specific talk show/ dj is getting a lot of viewers to let advertisers know that it's worth paying to put an ad on their station. Can they tell, like television, how many "listeners" there are? I doubt it because they just send out a signal that other devices pick up. Do they rely on surveys? Callers?
gpt
There are "consumer research companies" specializing in studying this issue for radio stations and other media companies. These consumer research companies, such as Nielson, do large surveys to determine what people are listening to, watching, and reading. They've benefitted enormously from the refining of "scientific" polling and have used it to make a successful business.

After the surveys, consumer research companies sell the information back to the media companies (radio, TV stations) who can then better determine where to spend their advertising dollars and which radio hosts, for example, are doing well with particular demographics.