Explanation of SP+
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.
The SP+ rankings generally perform well against the spread -- 52 to 54 percent success over a full season, which is excellent for a system projecting every game and not specifically adjusting for injury.
What goes into the projections?
The offseason projections -- which, like ESPN Stats & Information's Football Power Index, carry diminishing weight in the SP+ formulas throughout the season -- are comprised of three factors:
Recent history: I use a weighted five-year history as a way of gauging program health. For most programs, what they have done of late is what they will most likely do moving forward.
Returning production: Instead of using returning starters, I created a formula for returning production based on which units seem to have the most effect on a team's improvement or regression. As it turns out, continuity at quarterback, receiver and the secondary is far more impactful on SP+ rating the next year than continuity in other units. Accordingly, turnover in those areas takes on heavier weight.
Recent recruiting: Returning production measures what a program lost, and a two-year recruiting average fills in the gaps on what kind of talent and athleticism it has acquired to replace the lost producers.