system
Given the following post, respond as if you're commenting on Reddit:
human
How do radio stations know how many people are tuning in?
gpt
There is little fancy engineering or hard science behind this, because as you might imagine there is no direct way to tell that someone is receiving a radio signal and is actually tuned in / listening to a radio station.

Instead, certain companies compile the so-called "ratings" (which are a measure of the number of listeners) by sampling a small subset of the population. These volunteers will record their listening habits and provide demographic information (age, family composition, socioeconomic status, etc...) and with this data and the appropriate statistical tools, the company is able to make an estimate for the total number of listeners in a market.

In the past, listening activity was logged manually by the volunteer, who would simply write down when they would tune in to the radio and which station they would listen to. These days, it's more common to use a device which picks up inaudible tones embedded in the broadcast and uses this information to determine what station is being listened to and when.