system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do people know how many people watched an episode of a show?
gpt
In the US, a company called Neilsen gather the information either through users manually recording their viewing habits, or using a box attached to the TV to measure usage. 

Obviously not everyone has these boxes, so a bit of statistical jiggery pokery goes on to get the final result: the numbers are not exact.