While critics have praised changes made to the "CBS Evening News" since Bob Schieffer (search) took over for Dan Rather (search) last month, viewers clearly haven't been intrigued.

The show's average of 6.1 million viewers last week was the lowest since Nielsen Media Research began measurement with "people meters" in 1987 — and likely the lowest for many years before that. Evening news viewership was much higher from the 1960s to 1980s than it is now.

It was disheartening ratings news for CBS coming on the eve of the important May ratings "sweeps" and encompassing a relatively busy news week, with the election of Pope Benedict XVI.

"Our focus right now is on the content of the broadcast," said CBS News spokeswoman Sandra Genelius. "We realize it will take time for the audience to respond to the changes we're making. The journalism is strong, the spirit is fresh and the initial reaction has been very positive."

Schieffer has brought a more conversational style to the broadcast, and often engages CBS correspondents in brief question-and-answer sessions after they deliver their reports. Some critics have called that a welcome stylistic change after the intense Rather.

But Schieffer, 68, was appointed an interim successor with no clear indication of how long he'll be doing the job. CBS has also talked about dramatically changing the broadcast's format, but has given no indication of when this will be done.

CBS has also not given any additional promotional time for the show.

The show's average of 6.8 million viewers for the six weeks that Schieffer has anchored is down 7 percent from the 7.3 million from the comparable six weeks during 2004, Nielsen said. It's also down 10 percent from Rather's last six weeks although, to be fair, evening news viewership generally declines when the weather gets warmer.

NBC's "Nightly News," where Brian Williams (search) has replaced Tom Brokaw (search), is also down 7 percent in viewers from the same six weeks last year but remains the top-rated evening news broadcast. ABC's "World News Tonight" is essentially unchanged.

Within the 25-to-54 demographic that advertisers seek — most news viewers tend to be older — CBS is down 13 percent from last year, NBC down 7 percent and ABC up 2 percent, Nielsen said.

CBS hasn't given Schieffer, who continues to host "Face the Nation" on Sundays, a clear indication of how long he'll be commuting from Washington to New York to anchor the evening news.

But he's told CBS News he has his own deadline: he plans to retire on Feb. 25, 2007, his 70th birthday.

"I've made up my mind," Schieffer said in an interview with The Philadelphia Inquirer. "I want to leave while there's still a few people in the audience applauding."