Understanding Rebounds and the Celtics

Lately there has been much call and cry about how the Celtic's problems are because they can't rebound.

People look at the raw rebound totals ranking every team in the NBA and see the Celtic's at the bottom and jump to the conclusion that we must be a poor rebounding team.

That is a mistaken conclusion, but working from that point, they then conclude that if we are a poor rebounding team, and we struggle, our struggles MUST be because we are a poor rebounding team.

This is a case of starting with false premise (that raw rebound totals are comparable between teams) and leading on to another (the Celtics are a poor rebounding team) to then reach a third (poor rebounding is the source of our woes).

After the jump, I will try to explain why the raw rebound totals are misleading.  Further, I'll try to show how rebound rates lead to the basic philosophy behind the Celtic's strategy of play.   Finally, I'll end with pointing out how deviating from their strategy just killed them in their loss to Memphis (as well as a few other games).

The rebound totals are a bit misleading for several reasons.

First, let's summarize the Celtic's philosophy about rebounds.  This is straight from Doc's public statements and is easily confirmed by simply watching the team:

The C's do NOT, as a rule, go aggressively after offensive rebounds.  The C's prefer to instead have players hustle back down court on defense once the shot is up.  They DO fight hard for defensive rebounds. 

That's been the strategy for the last few years and worked well in getting us a championship.  In fact most really good teams put only token effort towards offensive rebounds.

Now, lets talk about rebounds, and specifically lets start with total rebound rankings.    Very often I read statements about how we rank against other teams in rebounding ("bottom of the league") which are simply missing context.

Yes, we are at the bottom of the league in total rebounds.  But that number is a product of several things.  First, it is the sum of both offensive and defensive rebounds.  We are dead last in offensive rebounds and 25th in defensive rebounds - reflecting the difference I talked about above.

But beyond that, you still have to look further.  A rebound itself is the product of two things :

  1. a missed shot and
  2. how good we are at grabbing that rebound.

In order to gather a lot of rebounds, BOTH numbers have to be high.

So before we can go comparing rebound totals, we have to look at how those two things compare.

Let's look first at offensive rebounds.

To miss a lot of shots you have to take a lot of shots and miss them.

The Celtics rank dead LAST in TAKEN shots (4794).   (Right off the bat that should get your attention more than anything else, but lets skip past that...)

We rank 4th in FG percentage (48.1%) and have often lead that category most of the year.  So our MISS rate is 51.9%.

So we end up having MISSED only 2488 shots, of which we grabbed 537 for offensive rebounds, a rate of ~22%  (the true rate is a little higher because I'm not accounting for dead ball after the (missed) shot due to the whistle being blown. I.E.the number of actual opportunities is actually slightly less than the miss count.  But for simplicity lets just go with this simple ratio).

For comparison, the #1 team in terms of total OFFENSIVE Rebounds is Detroit.  They shot 5182 attempts, missing 55.9%, or 2897 shots. So they took 400 more shots and missed nearly every one of them!  They grabbed 852 of those misses, or `30%.

Finally, lets look at our opponents - that is, how other teams did when playing us.  Our opponents shot 4902 FGAs (28th), missing 55.3% (8th worst) or a total of 2713 missed shots.  They grabbed 661 (22nd) at a rate of ~24%.  So as we <em>expected </em>our opponents ARE out-rebounding us on the offensive boards ... barely.

Let's next look at Defensive Rebounds.

Our opponents, as mentioned above, shot 4902 shots, and missed 2713 of them.  WE grabbed 1892 (25th) of them for a simple rate of ~70%. Again, the true rate is actually higher for the reasons given above.

Again, for comparision, the top defensive rebounding team by raw count is the Orlando Magic.  Their opponents took 5356 shots and missed 3019 (56.4%) of them.  The Magic grabbed 2182 of them for a simple rate of ~72%.  WOAH - not that different!

Finally, lets look at our opponents.  How well did other teams do when playing us.  We shot a league low 4794 shots and missed just 2488.  Our opponents grabbed 1847 of those misses for a rate of ~74%.   Again, as expected (because we are not fighting hard for offensive rebounds) our opponents are also slightly out-rebounding us for defensive boards.

Now do you see how each of these totals is off of a different pool of rebound opportunities?   OUR defensive boards come from THEIR misses, and vice versa.  Our DRBs came from a pool of 2713 misses.  The Magic's got theirs from a pool of over 3000 misses (because they allowed 454 more shots by their opponents!).  Our opponents grabbed their DRBs from a pool of 2488 - but were barely contested for them by us.

So the raw totals are vastly different and thus comparing rebound totals from team to team is meaningless.


Celtic's Basketball

Finally, one last comment about rebounds before we move on.  Looking at the above, you start to realize that when any rebound is in the air, the fact is that the defense is going to grab it 70-75% of the time.  *Period.* 

Next you can probably accept without me posting a lot of proof that when a team starts a possession with a defensive rebound, their chance of scoring is WAY higher than if they start by siding it in after a made basket.  The reason is simple:  A defensive rebound leads to a transition offense while the opponent still hasn't setup its defense. 

So everytime you miss on offense, there is simply a very high chance that not only will the other team get the ball, but they will be able to convert that into points.

So you have two jobs:

  1. Miss less. 
  2. And when you miss, get your butt back on defense.

THAT is Celtic's basketball.

And this is not new.  I posted this in another thread, but it bears repeating here:

Correcting for some of the things I ignored (dead ball after shot), this year's Celtics have a defensive rebounding efficiency of 74.1%.

In 2007-2008, this same starting personnel won the Championship with a defensive rebounding percentage of 74.7%

In 85-86, the Bird, McHale, Parrish Celtics (67-15, NBA Champs) were 17th out of 23 teams in Offensive Rebounding Percentage (31.3%). They were first in defensive rebound percentage at 71.7%.

I hope this provides a bit of perspective on the relative importance of the two rates.


Moving on:  Shot Attempts and Points.

If you look closely, what are the most important numbers up in all that?  Its not the rebound totals.  We've shown how they are not really comparable to each other.  Its not even the rebound percentages - those don't really show much difference (the defenses grab them at about 70-75% of the time) and also are against different pools of opportunity.

The most important numbers are how many shots we take and make and how many shots our opponents take and make!

THOSE are what get converted into points.

In a given game, each team has the ball basically the same number of times.  The possession total never varies by more than 2 and is usually identical.  So each team has the same number of possessions with which to convert into points.  The total NUMBER of possessions has mostly to do with the pace of play of the two teams.  A fast break team like the Suns will typically have over 100 possessions for each team each game.  The Celtics on average have about 91 possessions for each team.

Yet, if we and our opponents had the same number of possessions, how come we only shot 4794 FGAs, while our opponents shot 4902 FGAs?  That's 106 fewer shots than our opponents, or roughly 2 shots per game.

The first time I noticed this I assumed it was free throws or turnovers.  Maybe we were getting fouled more before the shot and a lot of them were shooting fouls.  But we've gone to the line 1591 times to our opponents 1602 times - virtually a tie in that category.

Also, while we ARE turning the ball over a ton (higher than most 'good' teams by 2-4 per game) we are also CAUSING our opponents to turn the ball over at a slightly higher rate.  Mostly that accounts for just about a half a possession per game in OUR favor - so we still need to find out how they are taking 106 more total shots.

Well, most of the extra shots go back to offensive rebounds.  They grabbed 661 of their misses to our 537 - so 124 extra chances to take a shot on their existing possessions. 

Wow - this must be killing us, right?  Well, not really.

This is where turnovers kicked in - we only allowed them to take 106 shots on those 124 extended possessions - even though most were probably close to the basket.

And this is where defense kicks in - even though our opponents shot at the basket 106 MORE times, they actually MADE 119 FEWER shots overall.

The original reason they are even GETTING a CHANCE at those extra 124 offensive rebounds is because we made them take poor shots and MISS the shot prior to each opportunity!

THIS is what the payoff is for NOT CONTENDING for offensive rebounds and instead getting back on defense and making the opponent work for their shot.  I.E. eliminate easy, high-percentage shots.

Conversely, our offense is predicated on creating high-percentage shots.  We do this two ways

  1. by using our defense to create missed shots and thus defensive rebounds as well as turnovers to fuel our own transition offense and
  2. in our half-court offense using our perimeter threats (Ray, Paul) to create floor spacing and then attacking the hoop for high-percentage shots.  We take a huge chunk of our shots in the paint and that is why we have the high FG%.


Is it working?

Most of the last few years as well as this year, this has worked.  Even in the Milwaukee loss, we only gave up 86 points.  But in that game, our offense stalled.  Pierce, Perk & Ray all collectively shot horribly.  The thing is, as dissapointing as that was, sometimes that's going to happen.  We still played the correct style of basketball - the shots just didn't fall.

It did not work in the Memphis game for a much worse reason.  Why?  Because in the first period we did NOT play Celtic's basketball.  First, we did NOT work to create high-percentage shots on offense and instead tossed up first-opportunity lazy shots which missed and of course were grabbed (70% of the time!) by Memphis and then - to compound it - we did not hustle back on defense when we did miss and thus committed the cardinal sin of letting the Grizzlies have easy easy easy access to the basket and uncontested 3s.  The Grizzlies shot at a ridiculous high % both overall and from 3PT land.

We were missing shots.  We were not in defensive position.  We were not contesting shots.  All that happened in about the first 12-15 minutes of the game and primarily with our starting 5 on the floor.

Somewhere shortly into the 2nd period, with the bench in there, we finally started playing 'normal' Celtic's basketball.  From that point on we played even with them, and slightly better in the second half.  But the damage was swift and deep and we couldn't begin to come back from it.

Again and again this year, we get into these funks where we can't seem to make a shot.  That by itself wouldn't be so bad - so long as we immediately get our butts back on defense.  But for some reason we have these 10 minute stretches where we let our offensive woes interfere with our defense - instead of letting our defense trigger our offense.

And when we do that, we get murdered.

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