Let's take a look at the 2011-12 season (the twenty-third (!) overall) of The Simpsons on Fox.
Rating the Ratings: Fox's looooong-running animated franchise turned in yet another perfectly fine season, dropping 11% and remaining the same borderline hit program that it's been for several years now. As usual, the show's high point (5.4) came in the one week when it got the massive NFL playoff lead-in, but the show's Truly most impressive performance was the annual Treehouse of Horror episode on 10/30/11 (a 4.0 with no NFL assistance). Things got fairly ugly in the spring, but that's normal for the Fox animation block; the year-to-year trends were for the most part not wildly worse at the end of the season. One notable exception was the show's plunge to a mere 1.9 demo on May 13 against the finales of Once Upon a Time and Survivor. Overall, I see no reason why it shouldn't jump right back up in the fall when it gets back into the NFL halo. Grade: C+.
Here's the now updated War of 18-49 post for The Simpsons.