SP+ rankings after Week 13: Let's get ready for Rivalry Week

After a week with only a couple of really big games, the top of this week's SP+ ratings haven't changed very much. In fact, the top 10 didn't change at all. Ohio State remained No. 1 after a mostly comfortable win over Penn State (which remained No. 7), and you have to go down to the near-teens -- Wisconsin moved up to No. 12, Memphis to No. 13 -- to find any movement.

Part of the reason for this stagnation at the top: SP+ never liked Oregon very much. The Ducks ranked just 15th heading into last week, and SP+ was far friendlier to Arizona State's chances (Oregon by 10.2) than Vegas was -- the line began around Oregon -15 before shifting toward -13.

That was the highlight of another strong weekend for these ratings. SP+ went 32-22-4 against the spread (59%), and after a midseason slump (49.7% ATS from weeks 7-9), it's caught fire again (56% since week 10) and remains at 55% for the season.

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.