Tuesday, April 21, 2009

The Value of Rebound Control

Dirk Hoag, who runs the excellent stats-focused blog On the Forecheck, has been measuring rebound shots from the NHL play-by-play logs over the last few years and was kind enough to provide me with a summary of rebound shots against by team for this season to date. I was particularly interested in this year's numbers since Martin Brodeur's absence should give us an idea of the impact of a goalie on rebounds against. You can't go very far on a New Jersey Devils blog or message board without running across someone either praising Brodeur's rebound control or trashing Scott Clemmensen's, so I think it is fair to say that one of the best at the skill was replaced for 40 games by one of the worst. Not to mention, Brodeur himself seems to think that rebound control is a pretty important skill. What was the difference for New Jersey in the mostly Marty-less 2008-09?

First, we need some context. The Forechecker has posted data online from each of the past several seasons (for example, here is a post from last March for the 2007-08 season). These data sets aren't usually complete, but they give us a pretty good sample to work with. Over the last three seasons, with Brodeur nearly always in net, The Forechecker has estimated the Devils as averaging 1.5, 1.4, and 1.4 rebound shots against per game. That is compared to a league average of just below 1.6, and is a good enough to rank 7th best in the league over that time period.

New Jersey Devils, 2008-09: 1.22 rebound shots against per game, 4th fewest in the NHL

According to the NHL play-by-play data, New Jersey took out one of the best rebound controlling goalies in the league and put in one of the worst, and not only improved their numbers from previous years but ended up among the league leaders in fewest rebound shots against.

How is this possible? Based on this result and other evidence, I think rebound control is an overrated skill. That is not to say it is unimportant, just that it gets overly emphasized in terms of its impact on goals against. This is for the same reason that most non-save goalie skills are overrated: Because the rest of the team can compensate for it. Clemmensen's teammates knew there were going to rebounds when he was in net, and they made sure the other team didn't get them. This shift happened without any apparent effect on the overall play - the Devils' rate of goals for, goals against, and shots against is very similar with Clemmensen and Brodeur in the net this season.

Rebound control is especially prone to being overemphasized because it is an obvious skill. You don't have to know much about goaltending technique to know whether a goalie is controlling his rebounds, you just need to watch where the puck ends up after it hits him. Lots of people who aren't goalies can speak knowledgeably about a goalie's rebound control. The problem comes when they focus too much on where the puck goes after a shot and not enough on whether or not it went in the net to begin with. A goalie who stops a high percentage of shots but has a tendency to allow awkward rebounds (someone like Tim Thomas, for example) will be consistently underrated by people who rely on subjective evaluation.

Despite the focus on rebounds, there really aren't that many rebound shots per game. This season there have been just 1.43 rebound shots against per team per game. Not only are the totals low, but there is not a whole lot of difference between teams in rebounds allowed. Over the last 4 seasons from The Forechecker's numbers, Detroit allowed the fewest rebound shots (1.25 per game) while Florida allowed the most (2.00 per game). That is not particularly surprising, since Detroit allowed the fewest total shots (25.6) while Florida allowed the most (33.1). Note that the difference in overall shots (7.5) is 10 times as high as the difference in rebound shots (0.75). Clearly it would be a big mistake to attribute a difference in shots against between teams primarily to rebounds.

A better measure of rebound prevention is the percentage of shots against that are rebound shots. The best team in the league this year, Buffalo, has faced a rebound shot on just 3.6% of their shots against. The worst team in the league, Carolina, has seen a second chance opportunity on 6.1% of their shots. That is a gap of 2.5%, which is a typical gap between the best and the worst in any given season. Even if we assume that the entire difference is a result of goalie skill, for a team with 30 shots against that accounts for a difference of about 0.75 rebounds per game, which at a typical rebound scoring rate is somewhere around 0.18 goals per game. A difference of 0.18 goals per game is equivalent to a save percentage difference of .006. That is just for this season, where we would expect some more randomness in the results. Over the 4 year sample, the difference between the best and worst is just 1.7%.

That would be a poor assumption, however, because it doesn't into account team defence. A better defence will clear more rebounds, and a team that allows fewer dangerous scoring chances will make it easier for the goalie to control his rebounds. Here are the correlation coefficients between team shot quality against and rebound shots percentage:

2005-06: 0.39
2006-07: 0.09
2007-08: 0.42
2008-09: 0.30

That looks to be pretty good evidence of an underlying relationship. If we take that into account, then that .006 gap might shrink to .003 or .004, and that is between the best and the worst. That's not even taking into account how good the defence is at clearing rebounds. Once you include that in the equation, there likely isn't a whole lot of margin left.

Is it possible that there is some other effect of rebound control that I am not missing? I brought up the possibility some time ago that goalies with good rebound control might be deterring shots, since opponents will be less likely to shoot from bad angles if they don't think they will be rewarded with either a goal or a rebound. That effect, if it exists, would be difficult to quantify.

These numbers are also subject to the limitations of NHL play-by-play data. If there is good reason to believe that more rebound shots and goals are taking place than are being counted, then we might have revise the strength of some of these conclusions.

One thing that may be possible is that poor rebound control may be an indicator of a goalie who is not on his game. It would be interesting to see if goalies are more likely to allow goals in games where they allow multiple rebound shots against.

I am certainly not saying that goalies should ignore rebound control or not try to develop their skills in that area. Excellent rebound control is of course preferable to poor rebound control, and will help prevent goals against. One of the reasons that we don't see a lot of difference in things like rebound control at the NHL level is that goalies with particularly bad skills in that area would never make it there in the first place. However, we still need to be particularly careful to avoid making the mistake of letting the obvious nature of rebound control overly influence our evaluation of a goalie.

15 comments:

Jon C said...

Great post.

I think that fans tend to overemphasize the impact of rebounds because it is a fearful thing to see a puck bouncing around in tight to the net with opposing forwards hovering.

However, a majority of the time those loose rebounds never get redirected back toward the net.

overpass said...

These numbers are also subject to the limitations of NHL play-by-play data. If there is good reason to believe that more rebound shots and goals are taking place than are being counted, then we might have revise the strength of some of these conclusions.Of course it's possible that the NHL scorers are undercounting or inconsistently counting these shots, but I think it's unlikely to affect your conclusions.

First, the low number of rebound shots basically refutes the charge that rebound control is a major cause of the difference in shots against between goalies. Since this charge is regarding the number of shots counted by scorers, not shots actually taken, scorer bias or error plays little or no part.

Second, it's possible that a few "jam it into the goalie" type shots on rebounds go uncounted. However, I expect that rebounds that result in any kind of a dangerous shot with a chance to score will be counted as shots. If you are looking for the actual value of rebound control, this should be fine over the long run. In actual fact, you are only really interested in rebound goals when it comes to value. Various ways of counting rebound saves will simply change the number of rebound shots and the expected scoring percentage.

Regarding the correlation between team shot quality against and rebound shots percentage - a small amount of that will be due to the fact that rebound shots against are high quality shots.

I'd like to see the correlation between defenceman size and rebound shots percentage, where defenceman size is a partial measure of crease-clearing effectiveness.

The Contrarian Goaltender said...

"I'd like to see the correlation between defenceman size and rebound shots percentage, where defenceman size is a partial measure of crease-clearing effectiveness."

An interesting thought. How would you define size in a single variable, by height or by weight?

overpass said...

I think I'd try lumping height and weight into a single variable, maybe by taking the cube of height in inches, dividing by a constant to put it at a similar range as weight in pounds, and taking the average of the two.

If I had to pick height or weight, I'd probably pick height. I think both are important, but weight is more variable over a career and may be less accurate than height. Of course heights are overstated sometimes, especially for sub 6'0 players.

This is getting more into the area of players than goaltending, but the importance of size for NHL players is an interesting topic. If the advantage of big players would show up anywhere in the numbers, you'd think it would be in front of the net.

I'm not sure this study would have particularly strong results, as Detroit has the smallest D in the league and seems to manage all right. Goaltending would also be a major factor. The effect of Chara clearing the front of the net may not show up if Thomas is allowing a lot of rebounds (and vice versa, it's hard to tell Thomas's rebound skill from the numbers when Chara is clearing the front of the net).

overpass said...

What percentage of rebound shots are goals? Given that, what is the range between the best and worst teams in rebound control in rebound goals allowed?

I think these numbers show pretty decisively that rebounds and rebound control have very little impact on shot counts, but it would be interesting to see the value of rebound control in terms of goals.

The Contrarian Goaltender said...

I was focusing more on shots than goals. I don't even the goals against data for 2008-09. I've been using 25% as a quick estimate of rebound scoring success, as I think last year Dirk Hoag's numbers had the rebound scoring rate at 28% of rebound shots.

There is quite a bit of randomness in seasonal rebound goals, since we are talking about only 120 or so events over 82 games for a typical team. If you read The Forechecker post I linked to from last year, the difference between the best and worst was 31 goals. That's certainly an unusually high gap, since the L.A. Kings had both far more rebounds against than normal as well as the worst rebound scoring rate against and ended up 9 rebounds goals ahead of everyone else.

Still, allowing say 10 fewer goals than average by protecting your crease would help win a couple of hockey games for a team over an entire season. It just seems to be a mistake to blame or credit the goalie for those goals saved, as the majority of the effect is probably determined by how the defence defends both the original shot and the rebound opportunities.

Anonymous said...

What exactly is used to define a shot off of a rebound. Again, this seems like a narrow definition, which does not account for very much other than a shot, and then the 2 second span between where it lands and who touches it. A goalie with good rebound control consistently redirects pucks where he wants them. This leads to easier breakouts, and less time in the defensive zone, not to mention fewer chances for the attacking team. This is contrary to a goalie who is bad with rebound control where a shot may be taken, and then recovered by the attacking. In neither case, no rebound shot occurs within the parameters of this study, however in the first example, the defending team gains control of the puck, whereas in the second case, the attacking team, while maybe not shooting directly off a rebound, regains control of the puck, and continues on the attack. If they score, lets say 30 seconds later, that shot and goal are then the result of that rebound.

The Contrarian Goaltender said...

I don't see why that goal is because of that rebound any more so than it was because of the failure of the defensive unit to defend their zone and get the puck out in that 30 second span. A Harvard professor did a study on puck location and puck possession and figured out that no matter where the puck is now or who has it, that doesn't have any effect on anything that happens 40 seconds or more in the future because hockey is a fluid game. See here.

If you want to hang your hat on goalies having a big effect on zone time, that's fine, there may be some truth to that. That's probably the only thing left that might show a greater importance for goalie rebound control. I'll get there eventually, and we'll see what the results say.

Jonathan said...

So assuming that 25% of all goals

The best team in the league this year, Buffalo, has faced a rebound shot on just 3.6% of their shots against. The worst team in the league, Carolina, has seen a second chance opportunity on 6.1% of their shots. That is a gap of 2.5% (or one every 40 shots)

Basically the difference between the best and the worst is one goal every 160 shots. This is very significant. The spread between the best and the worst overall SV% in the league is .040; that's one goal every 25 shots, or 6.4 goals every 160 shots. This implies that roughly 15% of a goalie's skill is based on rebound control.

"Over the 4 year sample, the difference between the best and worst is just 1.7%."

Over a four year sample, the spread between the best and the worst sv% by team shrinks from .04 to .02925, or a goal every 34 shots. 1.7% implies a goal every 235 shots. Again, these stats indicate a 14-15% impact of rebounds on goalie performance. Rebounds matter, albeit rebound control is definitely a secondary skill to simply stopping the first shot.

You might have the numbers on this, but just eyeballing it I imagine there's a pretty strong correlation between quality of defense and sv%. I'm just going off of the fact that the Minnesota Wild have the highest sv% in the league over the last four years. Anyways, it seems to me that statistically speaking, rebound control is an important aspect of an NHL goaltender's game. It's only one aspect, but it a significant one.

"Clemmensen's teammates knew there were going to rebounds when he was in net, and they made sure the other team didn't get them."
If they are focusing on rebounds more with Clemmensen in net, does that not imply that they are taking attention away from other aspects of defensive play, thereby hurting their overall defense?

The Contrarian Goaltender said...

Jonathan: It's not a bad argument, if the goalie was fully responsible for the number of rebounds against. I think it is pretty clear that is not the case, though.

On Florida, Scott Clemmensen would probably lead the league in most rebounds against. On New Jersey, he is in the top 5 fewest.

I'm not saying rebound control is insignificant, just that it is overrated. A good goalie might save a team 4-5 goals over a season because of his rebound control over a bad one, all else being equal, which is probably good for an extra win or two. However, the point is that usually all else is not equal, and that the rest of the team will adjust to a goalie that kicks out more pucks, narrowing the gap.

"You might have the numbers on this, but just eyeballing it I imagine there's a pretty strong correlation between quality of defense and sv%."

There is. There is a pretty strong correlation between quality of defense and every goalie stat.

"If they are focusing on rebounds more with Clemmensen in net, does that not imply that they are taking attention away from other aspects of defensive play, thereby hurting their overall defense?"

Not necessarily, since the results were pretty good, almost identical to the results with Brodeur. There are lots of different ways to play defence. I don't think the Devils were necessarily hurting their defence, just changing their coverage. From the shot quality data, it looks like they were defending a little deeper with the backups in, which means they allowed more outside shots and fewer rebounds and high-quality close-in chances. That might be an even tradeoff.

David Hutchison said...

Clearly the team in front of a goalie has a big effect on rebound shots. No questions there.

To use the Devils forums as your source to decide Clemmer is one of the worst rebound controllers is to go completely against your typical ways - hard numbers. But perhaps it was a bit tongue in cheek.

I would really debate the number of rebound shots as calculated. Pucks jammed at the goalie tight in after a save are not counted - I suppose because had the goalie not been there the original shot would have gone in? Not sure on that one, it's always been a bit of a mystery.

More importantly - I think you should look at puck possession much like a goalie playing the puck behind the net. i.e. a rebound may not result in an immediate rebound shot - but if it gives possession back to the attacking team shortly thereafter another shot could result that it not classified as a rebound shot.

Interesting analysis as always though!

Steven said...

I think the problem always with analysis you don't see the actual situations with the goalies. Clemenson has awful rebound control. He knows it. So to compensate, he got goalie pads that would kick out rebounds pretty far. As a result, many times opponents had to catch those rebounds much further out.

Simply counting the number of rebound shots would be erroneous in Clemmenson's case.

I watched every single one of those games, and Clemmenson's rebounds have often given possession back to the other team. In fact at times, NJ d-men could not clear the zone at all last year.

The reason Clemmenson's SV% was so high was because since nobody really seen him play, they had problems at first figuring him out. Once they did though, you saw a lot more goals on him. He's very good at saving the 1st shot so that was in his favor when his rebounds bounce so far back out.

Anonymous said...

WOW another brilliant post. Sense any sarcasim? "I think rebound control is an overrated skill" Thats your quote. Again you never cease to impress me with how little you know about goaltending and it makes me wonder why you have a hockey blog. As a goalie I actually know rebound control is a huge deal. Not being ready, in your stance and square to the shooter is a huge disadvantage to a goalie. Again your obsessed with numbers that dont mean shit. Ill tell you form experience you dont know what your talking about. Once I got to Tim Thomas I stopped reading. No one thinks hes under rated or bad. He was the backup for USA this Olympics. Your post might be from last year but I knew he was good then and before then. Your a joke. Just give it a rest.

Goaliemon40 said...

Just had to say I love seeing the posts here, good and bad.They all make us think a bit, and, in the end, hopefully we develop better goalies.
The other night I watched Brodeur kick a couple of rebounds out past the blue line and could just imagine the number of people out there thinking how terrible it was! In my mind it was brilliant. The puck was out of his end, and no chance for a rebound shot. As I think you mentioned, rebound control is not about absorbing every shot. It's about making sure the puck ends up where it won't quickly end up back in the net. In Brodeur's case, he even gets the puck to where there is an offensive opportunity for his team.
By the way, being a great goalie means being able to be an integral part of a teams defensive system. If you don't fit that system, the team probably won't pick you. So really, a great goaltender can adapt to a team's defense, and therefore, much of a goalie's ability can be measured by team success! I agree that it's not the only measure, but it certainly is a major part.

Host PPH said...

I think that it is quite interesting that he is trying to measure rebounds. I bet that he found interesting things with that kind of information.