Nielsen ratings have been relied upon since the 1920s. While the Nielsen company originally measured radion ratings using techniques such as phone and mail surveys, they now use innovative digital techniques to provide television ratings for over 100 countries around the world. This link from the Nielsen website provides invaluable tips for improving digital advertising effectiveness.
Television networks use Nielsen ratings in order to find out what shows to run, and at which time to run them. Results such as reach and frequency allow media networks and cable television providers alike to maximize show exposure. Reach and frequency - measures of how many people see a program and how often they see it are telling statistics in the television business.