news outlets published stories about the supposed "Bill vs. Bart" rivalry. "Bart Gets an 'F' (season two, 1990) was the first episode to air against The Cosby Show, and it received a lower Nielsen ratings, tying for eighth behind The Cosby Show, which had an 18.5 rating. The rating is based on the number of household televisions that were tuned into the show, but Nielsen Media Research estimated that 33.6 million viewers watched the episode, making it the number one show in terms of actual viewers that week. At the time, it was the most watched episode in the history of the Fox Network, and it is still the highest rated episode in the history of The Simpsons. The show moved back to its Sunday slot in 1994 and has remained there ever since. The Simpsons has received overwhelmingly positive reviews from critics, and it has been noted for being described as "the most irreverent and unapologetic show on the air." In a 1990 review of the show, Ken Tucker of Entertainment Weekly described it as "the American family at its most complicated, drawn as simple cartoons. It's this neat paradox that makes millions of people turn away from the three big networks on Sunday nights to concentrate on The Simpsons." Tucker also described the show as a "pop-cultural phenomenon, a prime-time cartoon show that appeals to the entire family." Run length achievements On February 9, 1997, The Simpsons surpassed The Flintstones with the episode "The Itchy & Scratchy & Poochie Show" as the longest-running prime-time animated series in the United States. In 2004, The Simpsons replaced The Adventures of Ozzie and Harriet (1952 to 1966) as the longest-running sitcom (animated or live action) in the United States in terms of the number of years airing. In 2009, The Simpsons surpassed The Adventures of Ozzie and Harriets record of 435 episodes and is now recognized by Guinness World Records as the world's longest running sitcom (in terms of episode count). In October 2004, Scooby-Doo briefly overtook The Simpsons as the American animated show with the highest number of episodes (albeit under several different iterations). However, network executives in April 2005 again cancelled Scooby-Doo, which finished with 371 episodes, and The Simpsons reclaimed the title with 378 episodes at the end of their seventeenth season. In May 2007, The Simpsons reached their 400th episode at the end of the eighteenth season.