The second week of training camp seems like as good a time as any to discuss offensive rebounding strategies. This week, ESPN Insider began rolling out John Hollinger‘s season previews. In his discussion of the Boston Celtics, Hollinger pointed out that the Celtics’ offensive rebounding rate in 2011-12 was the lowest in league history, and by a fairly substantial margin. Boston is at the leading edge of a lengthy trend toward teams sacrificing second chances in favor of getting back defensively.
TrueHoop’s Henry Abbott noticed, and wondered if teams aren’t better off following Boston’s lead and entirely abandoning the offensive glass. At Wages of Wins, Dave Berri responded by pointing out that there is little relationship between offensive rebounding and Defensive Rating.
Today came layer two of the analysis, as blogger Mystic looked at a more refined measure of offensive rebounding strategy and found that teams that pay less attention to the offensive glass tend to be better both on defense and overall. I think this latest study gets us about 75 percent of the way there.
Using the ratio of offensive rebound percentage to defensive rebound percentage, adjusted for league average, is a much better way to study the issue because it gets at the fundamental decision of whether to crash the boards or not. For example, the Chicago Bulls are one counterpoint to the “get back” argument because they combine one of the league’s highest offensive rebound percentages with its best defense. However, the Bulls are generating most of those second chances with their bigs while everyone else gets back on D. Chicago just happens to rebound particularly well, and they don’t appear among the leaders in offense/defense rebounding ratio.
The limitation of the study Mystic did is that it only looks at correlation, not causation. We don’t know whether getting back makes teams better, or better teams tend to play more conservatively and defensive-minded because they don’t need to take as many risks. To try to really isolate the causation, I looked at year-to-year changes in offense/defense rebounding ratio as well as team performance. I also limited the study to 2004-05 to the present to consider only the way the game is being played since the rules re-interpretations that opened up the floor for offenses.
This perspective tends to support my view that there’s a trade-off between offense and defense when it comes to hitting the glass. There’s a positive correlation between the change in offensive rebounding and the change in Offensive Rating (+.125) and a negative correlation with the change in Defensive Rating (-.141), indicating that as teams hit the offensive glass more from one season to the next, they get better on offense and worse on defense. Even though the correlation with Defensive Rating is slightly higher, the overall relationship to change in winning percentage is ever so slightly positive. The bigger takeaway is that it’s almost zero. The r^2 figure suggests that about 0.1% of the difference in a team’s record from one season to the next is attributable to the change in their offense/defense rebounding ratio.
The same trends hold up when we look at the extremes. The teams that increased their offense/defense rebounding ratio the most had better offenses and worse defenses and were marginally better; teams that paid less attention to the offensive glass had worse offenses and better defenses and identical winning percentages on aggregate.
The one surprise of the study was that it appears personnel may have more to do with offense/defense rebounding ratio than coaching strategy. The teams that increased their offensive rebounding the most generally added dominant rebounders like Zach Randolph (2009-10 Memphis Grizzlies) and Greg Oden (2008-09 Portland Trail Blazers). The 2009-10 San Antonio Spurs are the ultimate example. For years, they had eschewed offensive rebounding under defensive-minded Gregg Popovich, but the addition of DeJuan Blair made them much more likely to come up with second chances. The Spurs scored a little more, defended a little worse and were essentially the same team.
Ultimately, I don’t think there’s one right or wrong answer about crashing the glass. The best strategy for each team depends on personnel and style, and any number of approaches can be successful.