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For the first time in many years, a network tried to increase the number of viewers watching the evening news instead of trying to steal a bigger slice out of an ever-shrinking pie, said Mr. Hartman, editor of “BBC World News America.”

“There are people who love Katie and those who don’t love her, and that was a factor,” he said. “But it was the overall dynamics. There was a rock that we couldn’t move, and I don’t think it would have mattered who we would have put in there.”

Although Mrs. Couric will leave the evening news, she might not leave CBS. The CBS Corp. is a powerful force in the syndication business as owners of “Dr. Phil” and “Judge Judy,” and the upcoming departure in May of Oprah Winfrey will leave a huge void in the talk show marketplace. Through CBS-owned stations, the company could give a big head start to a show helmed by Mrs. Couric. Because of the sales calendar, such a show likely would not begin until fall 2012.

A syndication deal with CBS is seen as the only possibility that Mrs. Couric would continue as evening news anchor on a temporary basis past June, if she were to agree to stay during an extended search for her successor.

Other chief contenders for Mrs. Couric’s services are NBC and Telepictures. NBC, her old home, is not considered a big player in the talk show business. It tried and failed to launch a show for Jane Pauley, one of Mrs. Couric’s predecessors on “Today.” Telepictures is bigger in the marketplace, producing “Ellen” and a new show with Anderson Cooper debuting in the fall, both of which could take potential time slots away from Mrs. Couric.

Each of the companies has related news divisions where Mrs. Couric could have some visibility before starting a talk show at CBS, NBC or CNN, through Telepictures.