Final answer:
Nielsen measures viewing audiences in every television market during rating periods known as sweeps. These ratings help television stations determine which shows to keep in production and set advertising rates.
Step-by-step explanation:
The rating periods during which Nielsen measures viewing audiences in every television market are known as sweeps. Nielsen ratings help television stations identify shows and newscasts with enough viewers to warrant being kept in production and also to set advertising rates based on audience size for commercials on popular shows.