How can TV networks count their viewers?

Question

Like if they say that the CW has over 3 million viewers or that certain show has 3.2 million viewers. How do they know?

Answer 1

In the US and in Canada, the company that tracks what viewers are watching for television networks is the Nielsen Company. Each Nielsen participant is given a small black box, essentially a cable and modem that sends information about what the person or persons in the family are watching. In general, Nielsen only records what is being watched, and it is possible to have two programs in an hour count as “being watched” by television networks. If one watches one program for ten minutes and then switches to another program for ten straight minutes the data collected can become faulty in determining the popularity of programs on television networks.