Saturday, November 29, 2008

The Problem of Rebounding

There's understandably a lot of teeth-gnashing about the Hoyas' rebounding this year. It's been abysmal, or at least it seems that way to most people watching, including myself.

In addition, there's a perception that Thompson's teams have never rebounded well. A perception that the offense cripples rebounding; that Thompson's choice of personnel has left the team lacking in the ability to grab boards.

Has Georgetown under Thompson really been that poor at rebounding? What are the possible reasons why this would be true? Is it the result of personnel choices or strategic choices or both? If the team has rebounded well at some time, what was different?

How do you measure rebounding?

The most commonly used measure of rebounding is rebounding margin. There's a number of problems with this:
  • Rebounding margin mixes offensive and defensive rebounding. Rebounding margin nets offensive and defensive rebounds for both teams, but the problem is that offensive and defensive rebounding are related, but different skill sets. Good offensive rebounding tends to be the result of strong individual effort. Defensive rebounding is much more of a team effort because it is much more highly dependent on all players establishing proper position. Good offensive rebounders often rely more on athleticism; defensive rebounders on position. More importantly, players rebounding on defense have positional advantage -- and rebounding margin doesn't account for that.
  • Rebounding margin fails to compensate for differences in opportunity. There's an opportunity for a rebound on every missed shot. But not every possession ends on a missed shot. Some end on a made shot; some on a turnover. Since a team is more likely to get a defensive rebound than an offensive one, rebounding margin can make a team appear worse or better than it is simply because a team has more offensive opportunities than defensive or vice versa. For example, a team that forces a ton of turnovers will have less defensive rebounding opportunities and a lower rebounding margin than you'd expect. (Georgetown under Thompson would actually have an inflated rebounding margin over the years -- a negative turnover margin and better shooting than opponents means many more defensive rebounding opportunities).
  • Rebounding margin fails to account for pace. More possessions means more opportunities for rebounds. This will not make a good rebounding team look poor, but a faster pace will make a good rebounding team look even better under rebounding margin.
A much better statistic is rebounding percentage. It is simply the number of rebounds a team gathers divided by their opportunities to grab a rebound. It is split into offensive and defensive rebounding percentage so as to avoid the problems of rebounding margin. All the rebounding percentages here will be from unless otherwise noted.

What is a good rebounding percentage? Poor? Average?

In 2007-2008, the average offensive rebounding % -- the % of shots the offense got an offensive rebound on -- was about 33%. The best offensive rebounding team in the nation was North Carolina at over 42%; the worst was Air Force at 20%.

About 70 teams (of 341) were under 30%. Only about 60 were over 36%. So 210 of the 341 teams -- about 2/3 -- were within 3% either way of average.

For defensive rebounding, obviously the average is the reverse -- 67% is about the average. The best defensive rebounding team last year was Utah at 75%. Only two teams were at 60% or lower -- Maryland Eastern Shore and Kennessaw State. Just like offensive rebounding, 2/3 of the teams were between grabbing 64% and 70% of defensive opportunities. (Note: Pomeroy's stats state defensive rebounding % as opponents' offensive rebounding %. I've just flipped the numbers.)

Think about those for a second. On average, 1 in 3 rebounds are grabbed by the offense. How often is the scream "Box out!" heard at the first offensive rebound allowed? Most fans treat defensive rebounding like a save opportunity -- something that should be converted at 100%. It's simply not going to happen.

The rebounding percentages Ken Pomeroy calculates aren't adjusted for competition. So while we'd obviously expect Georgetown to be well above average in terms of all 341 DI teams, we're also facing a Big East schedule that most of those teams aren't playing.

So, has Georgetown really been that bad? Have they always been that bad?

This year, they're pretty bad.

More of the focus from fans has been on the defensive rebounding. Before the Tennessee game, the Hoyas were only grabbing 63% of defensive rebounds -- ranking 264th in the nation. The Tennessee game didn't help -- they only grabbed 56% of all possible defensive rebounds.

What's the difference between 60% and an average 67%? Let's say there are about 45 opportunities for a defensive rebound for the Hoyas in a game. That's actually been high for this season -- but even with 45 opportunities, the 7% difference there is about three rebounds a game.

Of course, we expect them to be better than average, but improvement here is likely around five points per game. That's huge over the course of the season, but it isn't the only reason we lost to Tennessee.

Just as disturbing is the offensive rebounding. Before the Tennessee game, the Hoyas were only grabbing 25% of our possible offensive rebounds, ranking 316th. The Tennessee game was an improvement at 30%, but still awful. Our previous season low was seven and half percentage points higher. A great offensive rebounding team would be 15% points higher.

So Georgetown is not a good rebounding team. But have the Hoyas always been that bad?

O Reb %
O Reb Rank
D Reb %
D Reb Rank

Some interesting notes:
  • The Hoyas haven't been a bad rebounding team under Thompson. Georgetown has been an average rebounding team overall during Thompson's tenure. Only two years have they been below average on either stat (of eight) -- in defensive rebounding in Thompson's first year and just slightly in defensive rebounding in the Final Four year.
  • They've been a good -- or great -- rebounding team twice. Offensive rebounding during the Final Four run was fantastic, and defensive rebounding the prior year was very good as well.

What has caused the mediocre defensive rebounding?

Well, one thing that hasn't caused it is the offensive system. It's amazing to me that people blame the mediocre defensive rebounding on the offense, but they do.

But several defensive choices could have caused it. And number one on the list has to be the usage of zone defense.

It is no secret that conventional wisdom states it is much harder to defensive rebound out of a zone. A player isn't guarding an individual player so it is simply harder to body up.

How much teams pay man to man and play zone is not publicly available information, so there was no systematic way to verify this or quantify this. But we can look at Syracuse.

Syracuse played the 2-3 almost exclusively over the past five years (and longer). From 2004-2008, which is the five years where we have complete rebounding numbers from Pomeroy, they have had a slew of quality rebounding personnel -- Craig Forth, Hakim Warrick, Paul Harris and Arinze Onuaku.

That has been reflected in their offensive rebounding percentage -- 38% over that time period. That ranked an average of 33rd in all of college basketball, despite playing conference games against NBA frontlines.

On the other hand, their defensive rebounding % was just 65% and ranked 240th. That's 200 teams of ranking difference and 8% difference from average.

The simplest explanation why is that they have played almost exclusively zone.

Unlike his father, Thompson has shown a willingness to play zone. More importantly, the Hoyas tend to play zone when they have a thin frontline -- when personnel already are weaker on rebounding. It is a way to protect big men from foul trouble, but it is not helping the rebounding.

There are other systematic choices that hurt rebounding.

One is releasing your guards and forwards on the fast break. This may be exacerbating the issue this year (Freeman has a ton of fast break points but not so many defensive rebounds), but was hardly an issue in prior years.

Another is the decision to funnel drivers to the shot blocker.
By challenging shots, the shot blocker is commonly out of position for the rebound. This enhances our FG% against but hurts on the back end. The other guards and forwards need to do a better job of rebounding if Monroe is going to continue to help. But overall, given how good our defenses have been, isn't this a trade off proven to be worth making?

What about personnel? There's no doubt Thompson likes to play players "down" a position. This is his fifth year, and for most of his time here, he's had a small forward playing power forward (Bowman and Summers) and a weaker rebounding player at SF (Owens, then Freeman).

What effect has it really had? Our defensive rebounding when Green was the center with Hibbert his only backup was awful, but the next year Thompson had his best rebounding team of his tenure at Georgetown.

Hibbert was still only playing 60% of the minutes, and Jeff was playing 80%, which means that only 40% of the time did we actually play them together and even less of the time did we have Bowman at the three for a truly strong rebounding team.

With Hibbert, Green and Bowman, we had three strong defensive rebounders.

The next year, with Summers replacing Bowman, the rebounding dropped off, and almost all the difference was in that Summers was awful where Bowman was a strong rebounder even from the three.

Was there something else going on, defensive scheme-wise, that made Bowman a better rebounder than Summers? Perhaps. But there's also a good chance that Bowman was simply a senior and Summers a freshman.

More importantly, Thompson has had a strong defensive rebounding team when he's had a rotation of enough strong rebounders to always have two of them on the floor at the same time.

When he wasn't had that, the rebounding hasn't been nearly as strong.

What about offensive rebounding?

In his first four years at Georgetown, Thompson's teams have average a ranking of 75th in the country in offensive rebounding. In 2005, with Bowman playing PF, the team ranked 92nd. Last year, with Summers at PF, the team ranked 132nd. Again, these numbers aren't adjusted for competition, so ranking in the Top 1/3 of basketball isn't bad.

And in 2007, Georgetown ranked eighth. Yes, they had an NBA frontline in Hibbert and Green. Roy in particular was an excellent offensive rebounder. But still, to grab 40% of our team's misses while playing teams like Pitt and Connecticut is fairly incredible, and it was an unsung driver of the team's amazingly efficient offense that year.

The team is not grabbing any offensive rebounds this year. Why not? It's most likely a combination of small sample sizes and a young frontcourt. More important is what is not driving it: the offensive system.

The traditional Princeton offense has usually involved abandoning offensive rebounds in exchange for getting back on defense. Since the teams that run it have historically been disadvantaged in speed and overall talent, that trade-off has made a lot of sense.

Thompson has never run a traditional Princeton, though. Not at Princeton; not at Georgetown. And at Georgetown, he hasn't given up on getting offensive rebounds.

The system, despite often placing big men on the perimeter, has not significantly hindered our offensive rebounding. Why would it start now?

If this drop in offensive rebounding is not just a small sample, it would seem to be personnel driven in two ways. Summers, though improving last year in his defensive rebounding, has never been a good offensive rebounder. Monroe so far has not been as good a rebounder as Roy. Though I suspect that will change.

The second way is in how the team's personnel scores. Offensive rebounds seem to often occur after help defense down low. If the Hoyas low post scoring and drives to the basket do not create those situations, the offensive rebounding will suffer.

The Future

Unfortunately, the future is somewhat bleak for rebounding. Rebounding woes seem to be personnel-driven. The team has successfully rebounded both offensively and defensively in the past, but when it has done so, it has done so when it has had a strong rotation of quality rebounders. The players needn't have been all traditional big men, but there needs to be at least enough quality rebounders to always have two of them on the floor at the same time.

Right now, the Hoyas only have one player who might be strong (Monroe) and a player who has played
the third player in a rotation (Summers). Monroe will likely develop into a quality rebounder by year's end, but the key seems to be if Vaughn or Sims can become a strong enough rebounder to play well when Monroe and Summers are out, and if they can become a strong enough defender and offensive player that the Hoyas can play less zone to protect Monroe.

It may not get any better the next couple of years. While Sims may develop into a strong rebounder, Summers and Monroe are not locks to return next year. With DaShonte Riley decommitting, that means Georgetown may go into next year with a possibly strong Henry Sims and Julian Vaughn, who was not a strong rebounder at Florida State. Even the year after, when Summers and Monroe are all but guaranteed to be gone, the only big man recruit is Nate Lubick, who will be a freshman.

The Hoyas may fill in some of the gaps with recruits who can come in and rebound right away. But if Summers and Monroe jump, get used to screaming "Box Out!"

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