By the Numbers: Defensive Rebounding

So I’m thinking a little bit about rebounding, and in particular how individual rebound rate seems misleading in terms of how it relates to team success on the glass.

This occurred to me because I was looking at Karl-Anthony Towns’ stats on the peerless Basketball-Reference.com, and noticed that his defensive rebound rate is 30.1 percent, which is by some distance the highest on the team. It’s also the highest of his career, and is the seventh highest percentage in the NBA this year.

And yet. Of the Wolves four primary big men, Towns has the worst on-off difference for defensive rebounding. When he’s on the floor, the Wolves snag 74.4 percent of the available defensive rebounds. When he’s off the floor, that number spikes to 84.8 percent. That’s almost beyond belief.

Among the other bigs, Taj Gibson, who shares the floor with Towns most frequently, is just barely negative on/off (77.4 percent on, 77.7 percent off,) while the two back up bigs appear to have a strong positive affect on the defensive glass. Gorgui Dieng is nearly +10 percent (84.1 percent on, 75.1 percent off) and Nemanja Bjelica sits at 81.3 percent on, 75.9 percent off.

In fact, Towns seems to drag whatever big he plays with down. Just a week ago, Bjelica’s on/off was much higher, but with Dieng’s injury he’s been seeing the floor with Towns significantly more, and his numbers in this area have regressed somewhat.

Overall the Wolves are 15th in the league in defensive rebound percentage, which isn’t bad except that with the league’s worst eFG percentage against, failing to capitalize on any opponent misses hurts, especially since they are giving teams yet another opportunity to exploit that league worst shot stopping.

(Aside: They are sixth in offensive rebound percentage, and here’s where Towns and Gibson shine; the team hits the offensive glass much more effectively with either of them on the court than off.)

So what’s the deal? It does not seem to be primarily about contested rebounds. Among players with significant playing time, Towns is 12th in the league in contested defensive rebound rate per nba.com. That’s the best on the team by some distance. (Fun fact: Second best contested defensive rebound rate on the Wolves? Shabazz Muhammad.)

There has been the sense that Towns often “block chases” or over helps, leaving him out of position for defensive rebounds. And yet he still grabs a ton of them. That would also suggest that his wing players are not effectively back-filling and helping on the glass when Towns rotates. I suppose this could be true, though the wings he plays the most with, Jimmy Butler and Andrew Wiggins obviously, sport the highest defensive rebound percentages among the Wolves non-bigs.

Is it possible that with Towns on the floor, other players have a greater tendency to leak out? Again, I suppose this is possible, but I’d need to see evidence.

Looking at his combinations, it might be the case that Towns gets particularly hurt when he’s out there with the second unit. The two-man pairings that reflect poor defensive rebounding numbers are when he’s out there with guys like Muhammad and Jamal Crawford. Tyus Jones and Bjelica as well, though this is somewhat confusing given Bjelica’s overall positive on/off when it comes to defensive boards.

Perhaps we need to give Gorgui Dieng more credit. It might not be so much a Towns problem (though if the team rebounded defensively at the rate they do when he’s on the court, they would be near the bottom of the league) as much as it is a reflection of the hugely positive effect Dieng has in this area. Dieng is a huge positive for defensive rebounding with essentially everyone he plays with.

On the other hand, last season, when Dieng was playing more than twice as many minutes as he has been this year, they were 20th overall in the league in defensive rebound rate, and Dieng himself was only marginally positive via an on/off measure.

And it’s possible, perhaps even likely, this is merely small sample variance. In fact, I’m convinced that a large part of it is just that. Still, even last season, Towns was marginally negative in defensive rebounding on/off. A star big man should be having a better impact in this area of the game, but we’re still waiting to see it from him.

In conclusion, I throw it to you, because I do not have a clear answer as to why this is happening. My best guesses are some combination of variance, playing a higher percentage of his minutes with backup perimeter guys and almost never with Dieng, and his own tendency to get out of position.

But I’m open to better answers.