To find out who is watching TV and what they are watching, the company gets around 5,000 households to agree to be a part of the representative sample for the national ratings estimates. Nielsen's statistics show that 99 million households have TVs in the United States, so Nielsen's sample is not very large. The key, therefore, is to be sure the sample is representative. Then TVs, homes, programs, and people are measured in a variety of ways.
To find out what people are watching, meters installed in the selected sample of homes track when TV sets are on and what channels they are tuned to. A "black box," which is just a computer and modem, gathers and sends all this information to the company's central computer every night. Then by monitoring what is on TV at any given time, the company is able to keep track of how many people watch which program.