Regular PFR readers will recall that we published college football SRS ratings every week last season. With seven weeks in the books, and the BCS opening rankings coming out tonight, it made sense to start up the project for 2010. So how do we come up with SRS grades for college football teams?
PFR has used the Simple Rating System to grade college and NFL teams for years. All ratings or rankings are meaningless without explanation, and the link above explains what the SRS tries to do. The SRS version that I'm implementing below is most useful to predict future results; the SRS is predictive, not retrodictive. That means the SRS will have no trouble at all ranking a team that's undefeated and beat a team with one loss behind the very team it beat. Why? One, because we know that one game is just one game, and never is conclusive proof that one team is better than another; and two, because the SRS weighs each game equally. Of course, sample size issues are always present here; while I've waited for seven weeks before presenting the SRS, we really need to see a couple more weeks of action before we can have full faith in this system. For now, though, maybe they'll make you rethink your perception of a couple of teams.
So how am I calculating these simple ratings?
1) For each game, 3 points are given to the road team (unless it's a neutral site game). After that adjustment, all wins and losses of between 7 and 24 points are scored as however many points the team won by. So a 24-10 road win goes down as +17 for the road team, -17 for the home team.
2) Wins of 7 or fewer points are scored as 7-point wins and losses of 7 or fewer points are scored as 7 point losses, except that road losses of 3 or fewer and home wins of 3 or fewer are graded as 0 point ties. So a 21-20 home victory goes down as a tie for both teams. This is not as drastic as it sounds, because the SRS ultimately is not concerned with win/loss records. There is no distinction between a win and a loss (you don't need to make such distinctions in predictive systems) except for when the game is close. So three 10-point wins scores +30, just as two 20-point wins and a 10-point loss scores as +30. However, three 3 point wins (+9 before the adjustments, +21 after) is worth more than two 10 point wins and a 1 point home loss (+21 before, +13 after).
3) Wins/Losses of more than 24 points are scored as the average between the actual number and 24. This is to avoid giving undue credit to teams that run up the score. Oregon bludgeoned New Mexico on opening day, 72-0, but that "only" goes down as a 46.5 point win. Why? Because the game was in Eugene (dropping it to +69) and the average of 24 and 69 is 46.5. However, in FCS/I-AA games, there is no run-up-the-score modifier. Why? Otherwise, the elite teams could beat the FCS cupcakes by 64 points and go down in this system. Major thanks to Peter R. Wolfe for providing the game scores.
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