Quoted from that report: "AM/FM RADIO METHODOLOGY: Audience estimates for 48 large markets are based on a panel of people who carry a portable device called PPM

Quoted from that report: "AM/FM RADIO METHODOLOGY: Audience estimates for 48 large markets are based on a panel of people who carry a portable device called PPM that passively detects exposure to content that contains inaudible codes embedded within the program content. Audience estimates from the balance of markets and counties in the US are based on surveys of people who record their listening in a written diary for a week. The estimates in this report are based on RADAR and the National Regional Database. RADAR reports national network radio ratings covering the US using both PPM and Diary measurement and it is based on a rolling one-year average of nearly 400,000 respondents aged 12+ per year. The National Regional Database reports national and regional radio ratings for individual radio stations using both PPM and Diary measurement. It is published twice a year and the annual sample is more than 600,000 respondents aged 12+. NOTES: Listening to HD radio broadcasts, Internet streams of AM/FM radio stations and Satellite Radio is included the Persons Using Radio estimates in this report where the tuning meets our reporting and crediting requirements. Monthly Time Spent Listening is derived by multiplying the weekly time spent listening estimate by 4.33 (the average number of weeks in a calendar month)." So 90% of people in a panel with a PPM (http://en.wikipedia.org/wiki/Portable_People_Meter http://en.wikipedia.org/wiki/Portable_People_Meter)