system
Given the following post, respond as if you're commenting on Reddit:
human
How do radio stations know how many receivers are listening to them?
gpt
They don't.

They do a poll.

They look at a map and say "there are 3.5 million people in the area where they can easily listen to this radio station, let's call 3,500 of them and ask them if they've listened to the station in the last week."

So they do it, and they find out that 173 of the 3,500 people have listened to the radio station in the last week.

So they figure that 173/3,500 = 0.0494 is the fraction of people in the area that listened to the radio station in the last week. Then they multiply by the total number of people in the area: 0.0494 * 3,500,000 ~ approximately 173,000 listening in the last week.

With something like cable TV, you can get direct data, because the cable box can report back to the cable company what channel is being watched, but with broadcast radio/TV, there isn't any effective way to tell.