system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : How does the TV viewership calculated ? How do they know I am watching the TV program ?
gpt
They don't.  There are a bunch of families who have a special box attached to their TV, provided by a company called Nielson Media Research.  The box transmits what you watch back to Nielson, and the company uses that data to figure out roughly how many people are watching.