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December 10, 2009

Gerald Wallace and Diminishing Rebounding Returns

Filed under: Uncategorized — Kevin Pelton @ 3:14 am

Apparently, BBP readers aren’t real big fans of Tyson Chandler. The biggest source of feedback I got on today’s look at Gerald Wallace‘s improved rebounding was the question of how much it had to do with the Bobcats’ personnel–and, most specifically, the trade that sent Emeka Okafor to New Orleans in exchange for Chandler. For example, this e-mail is typical:

I just wanted to bring up the fact that you failed to mention that Emeka Okafor’s now in New Orleans. Emeka has been the major rebounder in Charlotte the last few seasons and now that he is gone Gerald has less ‘in team’ competition on the glass and may feel the need to crash the boards more.

Indeed, Okafor is the better rebounder, but the difference isn’t enormous. Last year, Okafor grabbed 19.0 percent of available rebounds; Chandler is at 16.0 percent this season. Teammates do have a considerable impact on a player’s rebound percentage, however. Coincidentally, Jon Nichols recently revisited a study on the diminishing returns of rebound rate originally done by Eli Witus two years ago. Nichols reached a similar conclusion–teammates don’t affect each other that much on the offensive glass, but they do in terms of defensive rebounding. In basketball terms, there are a certain number of rebounds that the defense is going to get no matter what, and instead of fighting the offense for them, defensive players are essentially battling each other. So adding an elite defensive rebounder will cause his teammates to grab fewer rebounds, and vice versa.

(Nichols also looked for the first time at diminishing returns on other stats, all of which makes his column very well worth your time.)

One way we can look at how teammates have affected Wallace’s rebounding is to look at how the Bobcats as a team have rebounded with Wallace on the floor, numbers available on his player page. Charlotte is grabbing 32.4 percent of available offensive boards when Wallace plays as well as 74.7 percent of defensive rebounds. Both marks are excellent. If we subtract Wallace’s own rebounding, we see his four teammates have a 25.5 percent offensive rebounding percentage and 45.1 percent defensive rebounding percentage. How does that compare to last year?

2009-10    OR%    DR%    TR%

Team      .324   .747   .536
Wallace   .069   .296   .183
Others    .255   .451   .354

2008-09    OR%    DR%    TR%

Team      .294   .714   .504
Wallace   .052   .203   .128
Others    .242   .511   .377

The data rather neatly illustrates the diminishing returns theory. Wallace has improved his offensive rebounding even though his teammates have also been better on the offensive glass than last year. At the defensive end, his massive improvement has helped offset the decline of his teammates. What this doesn’t entirely tell us, however, is whether Wallace is having to grab more rebounds or his teammates are finding fewer to go around because he is grabbing them.

To shed light on that question, I tried a slightly different method–weighting the rebound percentages of each of the other Bobcats by their minutes played to average them. Here, it turns out the offensive rebound percentages are the same (5.6 percent both years) and the defensive rebound percentages very similar (13.5 percent this year vs. 13.7 percent for 2008-09). Drop-offs from the Okafor-Chandler trade and in the rebounding of power forward Boris Diaw (from 9.7 percent to 8.0 percent) are offset by replacing DeSagana Diop (15.8 percent) with Nazr Mohammed (19.0 percent) and fewer minutes for D.J. Augustin, who is a horrendous rebounder (4.3 percent last year, 3.8 percent this year). Based on this, it doesn’t appear his teammates have had almost any role in Wallace’s improbable rebounding improvement.

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