BOB GARFIELD: This is On the Media. I'm Bob Garfield.

BROOKE GLADSTONE: And I'm Brooke Gladstone. Last month, this headline in the Dallas Morning News: Fox News Channel Continues to Crush CNN. So, that's been conventional wisdom in the cable news biz for at least two years, and a quick search of On the Media's archives shows us parroting that same seemingly indisputable fact that Fox News was beating CNN in the ratings game. But Steve Rendall of the liberal media watchdog group Fairness and Accuracy in Reporting does dispute it. In an article titled The Ratings Mirage: Why Fox Has Higher Ratings When CNN Has More Viewers. Steve, welcome to the show.

STEVE RENDALL: It's great to be on, Brooke.

BROOKE GLADSTONE: So you say that Fox is not actually beating CNN in the ratings, or actually, to be more precise, you say that on any given day, more people tune in to CNN than to Fox News. So what's the difference, then, between actual viewership and ratings?

STEVE RENDALL: Actually the number that's usually reported in the media is a number that Nielsen Media Research releases for free. That number is the average number of viewers that are tuning into a show or a station at any typical moment. That number gives extra weight to heavy viewers -- viewers who tune in to a station and linger there for a long period of time. For instance, if you tune in to Fox in the evening, first you tune in to Special Report with Brit Hume; then it goes through that into Shepherd Smith's show and into the O'Reilly Factor. You are counted every minute. You can show up in the ratings as several viewers. But there's another number called the "cume" which is not published by Nielsen Media Research. They only release it to their paying clients, and it counts the total number of viewers who tune into a station or a show on a given day for 6 minutes or more. And it counts all viewers. And CNN has a higher cume than Fox.

BROOKE GLADSTONE: So, obviously, this speaks to the different way that those two channels are used.

STEVE RENDALL:One of the ad buyers I talked to told me that CNN is like news radio. People tune in for a short amount of time to get the news and the headlines, but they treat Fox like talk radio, where they tune in for longer periods of time to hear the personality-driven and appealing programming that's available there. Now this could change at any time, but as it stands now, on any given day, CNN has about 20 percent more individual viewers than Fox does.

BROOKE GLADSTONE:Now, you wrote in your piece that reporters who use the Nielsen numbers ought to explain that the data are not entirely reliable measures of popularity and that really the numbers serve no purpose for journalists or the public.

STEVE RENDALL: Well, yes, they're not sort of a democratic tally of who's more popular. They're collected by the Nielsen Media Research Center to provide very complex data to advertisers. This gets back to that question of heavy viewers and light viewers. Advertisers want viewers to hang around long enough to see their ads several times, but they're not interested in reaching the same viewer over and over again throughout the day. They'd like to reach more viewers. By the same token, advertisers don't want to advertise on stations where the viewers tune in for so short a time that they don't see very many ads. So it's a tradeoff. One vice president of CNN that I quote in the piece says: We'd like to have Fox's average. Fox would like to have our cume.

BROOKE GLADSTONE: But I think it's safe to say that Fox would love to have CNN's ad revenue, cause they're still making substantially more.

STEVE RENDALL:CNN is still making more money than Fox. It's because they have those lighter viewers that are a more valuable demographic. Lighter viewers tend to be younger, busier and more free-spending. I expect that Fox will surpass CNN at some point in cume, but that will not necessarily mean that they'll bring in more ad revenue, because Fox will still have a core viewership made up of those heavy viewers that are not as attractive to advertisers.

BROOKE GLADSTONE:So let's assume, then, that you're right --that Fox is trailing CNN in actual numbers of viewers, and that it's trailing CNN with regard to ad revenue. It nevertheless has the perception that it's trouncing CNN in the ratings. What's been the net effect of this perception?

STEVE RENDALL: Well, I think it's great public relations for Fox. In fact, there is one thing I quote in the piece where Brian Killmede who's on Fox & Friends, very successful Fox morning show, responding to somebody who has said that Fox leans to the right. He says "Well, the people made us number one, so what does that say about the United States?"

BROOKE GLADSTONE: What does Fox News have to say about your findings? Have you heard from anybody?

STEVE RENDALL:Well, I haven't heard any actual official response. I did get an email from a Fox V.P. who - it was very snarky, very sarcastic -- it said "Great piece. Sorry your data's all old. We're ahead of CNN now in cume." And I quickly wrote back to him. I said, "Well, if you are, I'd like to see the numbers." Those cume numbers, remember, are not published. They're only provided for paying clients like Fox. "Send me the numbers. I'd be glad to do an update." I haven't heard from him.

BROOKE GLADSTONE: Okay. Steve Rendall, thank you very much.

STEVE RENDALL: Thanks.

BROOKE GLADSTONE:Steve Rendall is senior analyst for the liberal media watchdog group Fairness & Accuracy in Reporting -- or FAIR. We called Fox News spokesman Robert Zimmerman about FAIR's research. He says Nielsen ratings are regarded as the key measure. Nobody pays attention to the cume. Meanwhile, on Wednesday, a new media war was joined on talk radio. [CLIP PLAYS]