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In preliminary national late-night ratings, Mr. O’Brien scored a booming 7 household rating, which was almost triple the number he had been averaging for months in that measure.

More impressive was the number for 18-to-49-year-old viewers — the gold standard for NBC because advertisers seek to reach that audience. There, in overnight numbers from the country’s 25 largest cities, Mr. O’Brien hit an extraordinary rating, a 4.8.

Not only would that be by far the biggest rating in that age group for any kind of show at any time Friday night (if it holds up as a national rating and it will probably decrease only slightly), it is also a better number than almost every prime-time show that has appeared on NBC this television seas

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