In his 15th week back as host of NBC's Tonight show, Jay Leno's ratings among adults 18-49 have dipped below those that Conan O'Brien garnered 15 weeks after he took over the show last year, according to Ratings for Leno's Tonight were also below O'Brien's Tonight when compared with the same calendar week a year ago, the website observed. Another bad sign for Leno and NBC for the first week since Leno's return, he was tied with CBS's Late Show with David Letterman among adults 18-49. He had been leading Letterman in the preceding weeks.