THE RADIO EQUIVALENT OF TV's Nielsen ratings, Arbitron ratings measure who listens to which radio stations at various times of the day. Paper diaries that Arbitron households fill out and mail in weekly have been criticized for years as inaccurate at best.

Now, Arbitron has introduced the Portable People Meter (PPM), an electronic device that records listeners' stations automatically. By the end of 2008, the units will be in 12 of the nation's top radio markets.

Already, the meters are shaking up long-held beliefs about radio listening habits. People don't automatically change stations during commercial breaks, Arbitron reports.

“They're finding through PPMs that radio is a fantastic medium to reach working people” says Scott Hamula, associate professor of integrated marketing communications at Ithaca College, in Ithaca, N.Y. “Working people have money, and working people buy homes.”

Not only will the information be more accurate, it will also be more frequent. With paper diaries, ratings were issued quarterly. In PPM markets, Arbitron distributes 13 reports a year.

With a more accurate method of measuring audience, builders should become more comfortable with advertising on radio than in the past, says Harvey Haddon, president of Chicago-based FireStar Communications.

“Paying for specific audiences is an acceptable expense,” Haddon says. “When you talk about drive-time markets, when [a station] says it gets 35,000 people in a certain age category in a particular quarter hour, you can be pretty sure the station is getting that.”