How can CNN have more total viewers when Fox has such a commanding lead in average viewers? Conventional industry wisdom is that CNN viewers tune in briefly to catch up on news and headlines, while Fox viewers watch longer for the opinion and personality-driven programming. Because the smaller total number of Fox viewers are watching more hours, they show up in the ratings as a higher average number of viewers.
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One of the reasons for CNN's lead in CPM, according to the buyers, is the advertiser preference for lighter viewers. Such viewers tend to come from the most desirable demographics--younger, busier, more free-spending--and because they're harder to reach with ads, the law of supply and demand drives their cost up.
One media buyer we interviewed analyzed the contrast between Fox and CNN in terms of programming and viewing habits, telling Extra!: "CNN is like news radio where people drop in for the news; Fox is like talk radio, where they stay longer for the opinion shows."
http://www.fair.org/extra/0404/fox-ratings.html