There’s been no shortage of media coverage on Katie Couric’s first days as a network evening anchor. First, there was the $10 million promotional campaign before her big September 5 debut. Then the barrage of news stories that followed looking at everything from her set to her hair to the daily horserace of who was leading the evening news race and who was making “a comeback” – one week in.

But now that there’s been some time to catch a breath, how is Katie Couric doing as the new anchor of the CBS Evening News? Three weeks in, there’s an argument to be made that her premier, at least initially, has shaken things up – or taken them for a rollercoaster ride.

As anyone who has followed the story knows, her debut drew 13.6 million viewers, the network’s biggest audience in eight years and an increase of nearly 80% over Bob Schieffer’s ratings when he stepped down.But by the end of the second week, Couric’s ratings had fallen and put CBS News in a virtual tie with NBC News though still in front of ABC News.

What do such numbers mean? To put them in some context, those are big moves, especially compared to the recent debuts of other evening news anchors

Consider the NBC Nightly News’s transition from Tom Brokaw to Brian Williams in early December 2004. Brokaw left NBC after 21 years at the helm with the cupboard full, finishing his career ranked #1. And the transition to Williams was considered generally smooth, even by NBC’s rivals at CBS and ABC. William’s sign off itself evoked a sense of quite progression:“We will continue to work each evening to earn and preserve your loyalty.”

By the end of Williams first month, NBC was still out front without much change in ratings– a position the network would retain until Couric’s debut in early September 2006, but would quickly recapture it just a week later.

When Bob Schieffer debuted as the interim anchor on the CBS Evening News on March 10, 2005, following Dan Rather’s 24-year stint in the chair, there didn’t appear to be much of an impact initially. After two weeks, ratings were down just one percent from Rather’s numbers, though it should be noted that Schieffer was able to attract on average, about 600,000 more nightly viewers than Rather during his 18-month tenure.

And the two highly-publicized anchor debuts that ABC’s World News has experienced in the last few years have been relatively quiet ones when one looks at audience data. Following the death of Peter Jennings, the network experimented with a two-anchor team of Elisabeth Vargas and Bob Woodruff in early January 2006. In the first few weeks the ratings remained essentially flat.

Following a severe injury to Woodruff in Iraq in January and Vargas’s February announcement that she was pregnant and leaving the show to spend time with her baby, ABC turned to Good Morning America co-anchor Charlie Gibson on May 23, 2006. Despite Gibson’s popularity as a morning anchor and a rebranding (the show dropped the word “Tonight” to become simply “World News”), the newscast has remained in second place where it has been firmly entrenched for some time now, without any notable change in overall audience.

What accounts for Couric’s more volatile numbers? Some have attributed the surge in her first week to a strong curiosity factor. There’s evidence for and against that theory. Surveys conducted by the Pew Research Center this summer suggested that Americans were more familiar with Couric than her ABC and NBC counterparts before she became the anchor. But, at the same time, viewers had not seen Couric as an evening anchor. Over at NBC, Williams had filled in for Brokaw for ten years. Gibson had also filled in during Jennings’ treatment for lung cancer.

Surely, more time is needed for the dust to settle. But for now, $10 million has created a bit of uncertainty in the world of the network evening news— and that’s something we don’t see very often. If CBS News President Sean McManus sought out Couric because she might bring new people to nightly news, the early evidence is that his gamble may have potential.