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Saturday, August 21, 2010

Rankings High: Scientific Proof that Preseason Polls Matter

This is a re-posting. Given today's AP poll, it seemed appropriate.

This post will be a bit technical, but bare with me. I have argued before (rather convincingly, I think) that preseason polls are somewhat effective at predicting the eventual national champ.1 This then begs the question--do preseason polls just predict or do they actually influence the final rankings?

Those who argue that preseason and postseason polls are independent say that any correlation between the two shows that pollsters made some good guesses about which teams will be good and which won't. Florida might not finish #1 in 2009, but I can guarantee that they'll finish in the top 10. It's also possible that the relationship is spurious-voters put Notre Dame too high and Utah too low at all times, be it pre-, post-, or mid-season.

There is another camp that argues that preseason polls influence final rankings. They point to the stepwise fashion in which teams move in the polls. It is usually controversial for a team to jump another team unless the second team lost that week, and therefore those teams that start on top have an advantage over those that need to jump them. It can also be hard to get noticed if you start outside of the top 25. Consequently, preseason polling gives some teams a head start.

I also think we should not underestimate the importance of the pernicious disease I call Neuheiselitus. Much like Eli Manning or Mall Cop, people can't seem to figure out that Rick Neuheisel isn't actually any good at what he does. It often takes a while for pundits to realize that some talented teams with high expectations aren't any good. On the other end of the spectrum is Applewhiteocious-just because they couldn't find a helmet that didn't cover his eyes didn't mean Major Applewhite wasn't twice the quarterback that Chris Simms could ever hope to be, and yet he had a hard time staying on the field. This is disease is alternatively called Flutiecoccus and is now plaguing Hyundai and Canadian bacon.

Whose right? To answer that question, I used regression to estimate the importance of different factors-win/loss record, strength of schedule, national prestige, and, of course, preseason ranking. Basically, by taking into account other factors that can influence a team's final ranking, I can isolate the unique influence of preseason polls on postseason results.

I've used data from 1994 to 2008 from AP Poll Archive. I first used regression to predict the final rankings using only the win/loss records and the strength of schedule. In the blue box, you see the R-squared is .78-this means that just using these four factors we can very accurately predict the final rankings. The green box shows the strength of the effects. Each win moves a team up the polls (closer to number 1) by 1.6 on average and a loss moves you down 3.4. That should seem about right. A tougher schedule also moves a team up in the polls-no surprise there.
Next, I add prestige factors-total wins for the program, national champions and whether or not they are in a BCS conference. Of these, only being in a BCS conference really matters (if the number below P>|t| is above .05 the factor is not significant). On average, a team in a BCS conference will finish about 5 spots higher than another team not in a BCS conference with the same record and strength of schedule. Figures.
Next, I add general measures of the team's performance. the PerfRating is based on margin of victory and EloRating just on win/loss record (like those used for the BCS computer rankings). The EloRating is not significant because it measures the same thing as the win/loss record and strength of schedule, but the PerfRating is important. Finally, I add the preseason rankings. You will notice that the P>|t| value is below .05, which means that preseason polls have a real influence on postseason polls. In other words, the results in the final rankings would be different if we didn't do preseason polling. But before we get too excited, it is important to also look at the coefficient (=.0539). One spot in the preseason poll moves a team up .05 spots in the final poll. Or, being ranked 5th instead of 25th will move a team up one spot in the final rankings. So, while preseason polls do inappropriately influence final rankings, the effect is not large. It is marginal at best compared to conference affiliation, for example, which can be worth 4 spots in the final rankings.
One group does benefit more than others. The table below lists the biggest benefactors of preseason polling. The Pred. is where the team should have finished, but they all finished between 2 and 5 spots too high even after accounting for performance, wins and losses, SOS, conference affiliation, etc. They also have some other commonalities. They are major programs from BCS conferences. They were ranked between 2 and 5 before the season started and finished between 9 and 28 - in other words, they were thought to be national title contenders, but were, in fact, major duds. Classic cases of Neuheiselitus
In summary, preseason polls do influence final results in a way they are not supposed to, but not enough to really worry about. And teams in BCS conferences can lose one more game than an otherwise equal non-BCS team and still finish higher in the polls. Non-BCS conspiracy theorists have been right all along.

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