system
Given the following post, respond as if you're commenting on Reddit:
human
How do TV ratings work?
gpt
Nielsen is the main company that publishes TV ratings.  They recruit people to volunteer to have their TV viewing habits monitored, and in exchange, Nielsen pays them.

TV monitoring happens a few different ways.  The low-tech method is a "TV journal" where everybody in the household writes down what they are watching and when (coincidentally this is still how radio ratings are done).  Most ratings are now recorded by more advanced systems.  Usually there is a box that attaches to each TV with a series of buttons corresponding to each person who lives in the house.  Whenever a person is watching TV, they press their button.  This gives a much more accurate sample of what people are actually watching.

Nowadays, live + 7 is the normal standard for ratings.  This means that everyone who watches a show live or who DVRs the show and watches it within 7 days counts in the ratings.  The day to day ratings are somewhat meaningless (just channels competing with each other for bragging rights).

Ratings become very important in selling ads, though.  This is done mostly through the upfront sell which happens in April/May.  The ratings during "sweeps" season, when shows pull out all the stops to entice people to watch, are very important, but average viewership, demographics of viewers, etc. is also important.