Friday, November 4, 2011

The Impact of MOV Cutoffs in Football Ratings

I was prompted to start my football predictions by a discussion on an email list of the value of MOV cutoffs in rating systems.  Roger Dendy believed that capping the MOV in blowout victories improved his rating system.  My testing of MOV cutoffs in basketball has shown just the opposite -- that no matter how big the blowout, there's always information in the margin of victory.  Capping MOV at any level (in both blowouts and nailbiters) always reduces the prediction value of a rating.

Of course, just because that's true in basketball doesn't mean it's true in football.  I was pretty sure it was true, but I believe in "trust but verify."  So I put together the football predictor and tested a couple of different rating systems both with and without MOV caps.

I have many rating systems that use MOV, so I picked one and measured it's performance with a 100-fold X-validation  across my archive of college football scores from 2005 to date.  It had a RMS of 16.78 and predicted 71% of the games correctly.

Then I experimented with adding a cutoff to the MOV.  I set the cutoff to 32 points, so that all the games where the MOV exceeded 32, it would be treated as 32.  I just picked 32 arbitrarily as a good figure for a blowout win.  The performance degraded to RMS=17.19 and 69%.  I then bumped up the cutoff to 48 points, and the performance was RMS=17.01 and 70%.

The other rating system showed a similar pattern of performance.

What this shows -- at least for the two rating systems I tested and these performance metrics -- is that even huge margins of victory have value in assessing future performance.  People argue intuitively that there's "no difference between winning by 48 and winning by 52" but that appears not to be true.

Recently I got to wondering if it might not make more sense to drop a blowout victory entirely.  This would be like "drop your lowest score" grading in high school.  The intuitive notion here is that sometimes teams just have a bad day -- a few unlucky bounces and worse goes to worse.  Or lucky bounces and better goes to better, from the other side of the coin.  More on that notion next time.

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