Breaking: TVNewser has learned ABC is asking Nielsen to investigate their "World News" ratings for Friday - the day of the DTV switch. Insiders tell us Friday's Total Viewer number was 4.09 million - that's off from a 7.34 million Total Viewer average the show had been putting up on Friday's this season.

Two of ABC's strongest stations in the two of the four largest U.S. markets - WLS in Chicago and WPVI in Philadelphia - experienced reception problems after the DTV switch.

Taking out Friday, as well as Tuesday and Thursday due to the NBA Finals, ABC ends up with a 7.32 million Total Viewer average.

And just a week after the CBS Evening News with Katie Couric hit a new low, TVNewser has learned Couric's 5-day average for last week was 5.4 million - a 220,000+ viewer advantage from the prior week.

Final numbers still to come, but we hear NBC Nightly News with Brian Williams will again finish first with a 3-day average (taking out Tuesday and Friday for NHL Finals) with an 8.27 million Total Viewer average.

> More from ABC: "While there may be several issues at play, Nielsen has identified at least one problem - a certain number of ABC affiliates switched over from analog to digital before the midnight deadline and their viewership was not credited properly.
Therefore, Nielsen will be reissuing the Friday Primetime ratings as well as the figures for World News."