Sunday, October 17, 2010

Random Chance in the NFL

Normally over the course of an NFL season power rankings regress towards the mean and extreme ratings get closer to 1. This season however, the best and worse ratings are already closer to 1 than last season's final ratings, and we haven't had any undefeated teams for a couple of weeks. Has NFL parity really increased?
One way to judge parity might be to look at the percentage of time the better team wins. With no parity, the best team in a given game would win 100% of the time, being far better than it's opponent. But with perfect parity the better team would win only 50% of the time, and NFL game would be random chance. It's obvious that chance plays some role because otherwise the Rams would never win a game, but how much of a role it plays can be uncertain. To look at that, I'm going to look at what percentage of the time the best team in a game wins.
The key to my search was to look at variance in win numbers across the NFL. If all teams won 50% of the time, then winning percentages would be clustered around .500, giving a small variance. On the other hand, if the best team always one there would be undefeated and winless teams along with other extreme records, leading to a high variance.
Through 5 weeks in the NFL, variance per team was 1.24 wins per game. That matches up with the better team winning 62% of the time. To confirm, I looked back at the previous games to see how often the higher ranked team won. Turns out, the higher ranked team won 63% of the time, so it's pretty clear that in the NFL this year the better team wins somewhere around 62% of the time.
For comparison, I also looked at the last three years by variance. In 2009 the better team won 78% of the time, and in both 2008 and 2007 the better team won 80% of the time. If the Chiefs and Bears hadn't already made it clear, statistics show that this year parity and random chance have reigned over the NFL, so much so that picking winners is basically a crapshoot. At least that explains why my rankings systems can't pick NFL games. Also, it may give the Bears a chance at the playoffs. I kinda like this random chance thing.


  1. Not going to lie I got on your blog today to tell you to write about how the competition in the NFL seems to be closer than ever this year. I think that this is the first year since 1985 a team hasn't made it to 4-0 so it seems that this season does appear to be an outlier. Anyway good work with this one. I'm nearly 70% sure there is a conspiracy involving Vegas because they are making a killing this year.