College football Week 4 SP+ rankings

The combination of a couple of jarring upsets and the SEC's presence at the party made Week 4 of the college football season feel like the first true college football weekend of the year. And there was a decent amount of fallout from it in the latest SP+ rankings.

Below is this week's piping hot batch of SP+. As with last week, I'm now including the latest FBS teams that have committed to playing this fall -- the Pac-12, MAC and MWC all announced plans to jump in the pool this past week -- even though none of them will play for a few more weeks. That means we continue our streak of a new denominator each week: all 130 FBS teams were included in the preseason rankings, then 77 after Week 2, 91 after Week 3 and now 126 after Week 4. Nothing about this season is normal, but in theory we might stay at 126 for a while.

What is SP+? In a single sentence, it's a tempo- and opponent-adjusted measure of college football efficiency. I created the system at Football Outsiders in 2008, and as my experience with both college football and its stats has grown, I have made quite a few tweaks to the system.

SP+ is intended to be predictive and forward-facing. That is important to remember. It is not a résumé ranking that gives credit for big wins or particularly brave scheduling -- no good predictive system is. It is simply a measure of the most sustainable and predictable aspects of football. If you're lucky or unimpressive in a win, your rating will probably fall. If you're strong and unlucky in a loss, it will probably rise.