Adjusting Individual Defensive Efficiency Ratings
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A couple of months ago I presented individual defensive efficiency ratings for the 2008-09 regular season that I extracted from play-by-play data. In this post I will present a method for adjusting these ratings in an attempt to get a clearer picture of a player’s defensive abilities.
Adjusting the Defensive Ratings
To adjust these defensive ratings I fit a multilevel model that allows us to measure the individual offensive, individual defensive, and team defensive impacts on individual efficiency ratings. I fit this model for each of the 2006-07 through 2009-10 regular seasons, and I also fit a single model using all data from those seasons. The results of these fits can be found in the following spreadsheet:
In this spreadsheet you will find tabs for each of these model fits. The ratings are in terms of the player’s difference from the average defender. Standard errors are listed along with color coded confidence levels. These color codes give us an idea as to how much confidence we have in the estimate. In other words, green means we’re confident the player is not average, red means we have little confidence the player is not average, and yellow is the middle ground between the two confidence levels.
Interpreting the Ratings
To interpret these ratings, you have to think in terms of knowing the defensive player used the possession. For example, Dwight Howard’s 2009-10 rating suggests that when he uses a defensive possession the individual offensive efficiency rating of the player that used the offensive possession is 14.7 points lower than what it would be against an average defender.
It is important to note that because this model shrinks estimates to the mean, bad defenders that get little playing time will be considered average.
These ratings also adjust for the team the player plays for, as Dean Oliver shows in Basketball on Paper how some good defensive players can play on poor defensive teams. The general idea was to try and account for “Dumars-like” players while at the same time control for the idea that one individual doesn’t have complete control over how well a team does defensively.
I haven’t done anything scientific to fully study the impact of this team adjustment, but it seems to make sense after eyeballing the impact of this on players like Pau Gasol and Chris Bosh. Eyeballing something doesn’t give us a ton of confidence, so this adjustment is worth a deeper look in the future.
Players Still Underrated After Adjustment
These adjusted ratings do little to account for the fact that we don’t have a great way of giving credit to defenders when opponents make or miss shots. Guys like Shane Battier that defend the opponent’s best offensive player aren’t going to stand out in these ratings.
What Makes Sense? What Doesn’t?
I’m still trying to learn what makes a good defender, so I’d like to hear your thoughts on what ratings make sense, and which don’t. What players have reputations for being good defenders that this model isn’t estimating well?