The radio world runs on ratings. 

A “rating” is the percentage of potential listeners who are tuned into a program or station at a given time.

In 2010, the Indianapolis market began using the Arbitron Portable People Meter. The Portable People Meter™ (PPM™) is a unique audience measurement system that tracks what consumers listen to on the radio, and what consumers watch on broadcast TV, cable and satellite TV. It works by detecting identification codes that can be embedded in the audio portion of any transmission.

PPM technology has been adopted for use in media measurement applications in Canada, the Netherlands, Belgium and Singapore. PPM audio encoding technology has also been licensed for radio and television audience measurement in selected countries in Europe, Asia-Pacific, the Middle East and Africa. PPM replaced an out-dated diary system in which listeners manually wrote down the station and listening duration.