Tuesday, January 09, 2007

NFL Refs: Bias May Exist Regarding Home Teams but not for Favorites

I am now working on studying whether there are NFL referee tendencies either in favor of or against home teams or favorites. Are some referees better for home teams (or for favorites)?

The first question is how to come up with a model for predicting which team is the favorite. Please post comments to offer suggestions.

I checked how often the home team actually won to how many times you'd expect the home to win considering the teams involved. To estimate the expected-wins, I used the difference in season DVOA (using footballoutsiders.com statistics) and a homemade conversion of the difference and the estimate win rate.

I checked whether there was a correlation by comparing a random sampling of 6 out of the 17 referees and compared the first and second halves of the season.

The result: there may be a bias for or against the home teams by referee. But there might be no bias for or against favorites by referee. Specifically, there was a correlation of 0.177 about whether a ref favors the home team and a correlation figure of -0.162 about whether a ref tends to like the favorite or not. It was critical to factor in the strength of the visiting and home teams -- if you ignored that, there was no correlation. In fact, there were wild fluctuations about the expected win-rate of home teams for a ref in the first and second halves of the season (Houcholi had 63% in the first half and 40% in the second half while Winter had 49% in the first half and 63% in the second half...) This suggests you actually do want Mike Carey as your referee if you are the home team!


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