Polls, third party companies conduct polls of radio listeners, asking them who/what they listen to, and use their answers to make statistics and them used those statistics to extrapolate over a larger area who are listening. Radio stations can then use this information when trying to land advertisers.
to put it very simply, for say Radio Station X, the Polling company will survey 100 people randomly, and after that survey they find that 40 dont regularly listen to radio, 50 do but not station X, and the last 10 Regularly listen to station X.
So, in this group, 10% of people listen to the radio station, so if the total population of the area is say 20,000, you can be relatively confident that 2,000 regularly listen to Station X.
This same process can then be done with Shows within the Channel like Radio Stations within the area.
Stations can’t really know for sure how many people are tuned in at one exact moment
tl:dr/Eli5: They randomly survey their listeners and ask when/what they typically listen to.
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