There was some spirited debate attached to my original post about the disappearance of Conan's Tonight Show adults 18-49 lead over Letterman's Late Show and whether it might be the start of a long term trend. That was seven weeks ago and the two shows are still neck and neck in the important adults 18-49 demo.
This is close to looking like a permanent sea change. Not quite as dramatic as when Leno took the late night ratings lead from Letterman in the mid 90's and never gave it back, particularly because Letterman's unlikely to stay in his chair too much longer, but a big change nevertheless.
The trend lines above are 4 week moving averages.
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Nielsen TV Ratings Data: ©2009 The Nielsen Company. All Rights Reserved.