Tuesday, December 26, 2006

NFL: division rival games no closer than out-of-division games

Another nice Doug Drinen study, this time checking conventional wisdom on whether NFL teams in the same division play each other more closely than you'd expect given their records. That is, can you "throw the records out the window" when a mediocre team plays a better division rival?

Drinen lets you "draw your own conclusions." My conclusion is that the records of the interdivision games are pretty much the same as the records of the intradivision games. That is, you can't "throw out the records" because they mean the same thing, regardless of which divisions the teams came from.

For instance, when two teams five games apart in the standings (e.g., 7-3 vs. 2-8) face each other, the results for the team with the better record are:

.737 when teams are in different divisions (101-36)
.707 when teams are in the same division (99-41)

The effect goes in favor of the "throw out the records" theory, but it's pretty small. And, besides, 5-games-apart is only one of many categories. For a 4-game spread, it goes the other way:

.644 when teams are in different divisions (163-90)
.716 when teams are in the same division (184-73)

Skimming the various categories, you'll find that some go one way, some go the other way, and none are way off. You can see the full results at the above link. (And, by the way, only games from week 6 and on were included in the study.)

If you don't trust your eye, it wouldn't be difficult to do a statistical test on the two lists – that
Chi-Squared thing where you add up the "(expected-actual)^2 / expected" comes to mind. I'm pretty sure you'd find the difference to be far from statistical significance.

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At Wednesday, December 27, 2006 4:16:00 AM, Anonymous Anonymous said...

Highly. With 22 degrees of freedom, the threshold for statistical significance at p = .05 is a chi-square value of 33.92. If we do a chi-square test weighted by intra-division games (that is, the expected values are the intra-division results), we get a chi-square value of 9.08. If we weight the results by the inter-division games, our chi-square value is 10.82. Either way, highly insignificant.

At Wednesday, December 27, 2006 9:39:00 AM, Blogger Phil Birnbaum said...

Thanks, David.


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