Wednesday, July 29, 2009

Do Defensemen influence Save Percentage?

[Note: this methodology is similar to a previous analysis of replacement level: It is obvious that teams do worse when their top players are out of the lineup, but it's not clear what suffers.]

In order to answer this question, I made a list of defensemen over the last four seasons who averaged more than 22 minutes per game and played between 20 and 70 games in a given season. 22 minutes of TOI corresponds approximately to the 60th-best defenseman, which means that the group of subjects includes mostly 1st and 2nd defensemen.

I then counted up the number of even-strength goals and shots against when each defenseman was in the lineup and when he was not dressed. I further split the defensemen into those whose +/- was better than the team's overall +/- (on a per minute basis). Presumably the group with the "Better +/-" would have more of an impact on even-strength defensive zone play.

The results:

N=28 Better +/- 1610 1.85 21.48 914.0 686 2.04 21.94 907.2 5.8
N=25 Worse +/- 1229 1.89 22.20 915.1 821 1.89 22.00 914.2 0.9

The assumption here is that during a given season, a player's aggregate group of teammates, including the goalie, will be basically unchanged whether he's in the lineup or not. Similarly, his team's aggregate group of opponents is assumed to be the same whether he's in the lineup or not.

While the defensive difference with the "Worse +/-" defensemen out of the lineup was negligible, the teams of the "Better +/-" defensemen allowed 15.5 fewer goals per 82 games when they were playing. One-quarter of this change was due to reducing the volume of shots allowed, but the rest is due to better save percentage.

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Might the players in group with the better +/- be there *because* of the better save percentages? Both the +/- and the delta are a partly a function of the IN SVPCT.

If you lump both groups together, you do still get a delta of 3.4. Don't know whether that's big enough to be significant.
This is true. Indeed, 12 out of 28 defensemen saw worse save percentages when they were in the lineup than when they were out of it. This is not a large sample size.

But when you drop to a replacement level defenseman, your team does indeed do worse:

So it has to come about for some reason - most of it is reduced offense when your #1 D is out of the lineup.
My biggest concern is whether or not this is consistent from year to year in individual players. Are there individuals that are able to consistently outperform the team save percentage? If there are, it would seem that we are measuring a skill. If there are not, it would seem that we are measuring luck. I think we're probably measuring luck, which is still helpful for evaluating a player's talent level but in the opposite way, i.e. Defender A has a high on-ice save percentage and good plus/minus so he is skilled at forcing low quality shots vs. Defender A has a high on-ice save percentage and good plus/minus so we can expect that he will come back toward the mean in those areas in subsequent seasons.
I don't think that luck explains a 907 SVPCT when the defensemen are out of the lineup. I would expect that the bad +/- players would have had bad goaltending luck while they're in the lineup, not that the good +/- players' teams would have had bad luck when they were out.

Of course, as with many things in sports, none of this is significant even to one standard deviation.
I too would expect that the bad +/- group probably did, on average, have bad luck with regard to on-ice shooting percentage. But, if that's the case, then why wouldn't the defensive impact for this group be the same or similar when they aren't in the lineup as the good +/- group? When they're out of the lineup and replaced by a low level player, the team Sv% stays high (higher in fact than the good +/- players). That's what seems so odd to me. One group of replacement level guys is doing significantly better than the other. Why do you suppose this might be?

I think it would be good to test the assumption above. Maybe the group of 22+ min players with the poor +/- haven't been unlucky as a group. Maybe their coaches had better available options that they were underutilizing and, as a result, when those players are elevated, we don't see a drop in Sv%.

The other possibility is that the guys in the good +/- group might be, on average, important penalty killers (which would generally serve to increase their +/- and increase their effect on Sv%) while the bad +/- group might be, on average less important penalty killers, but used a lot of the PP (which would generally serve to decrease their +/- and decrease their effect on Sv%).

Do you have a list of the players that you used for the last two years. If so, I'd like to take a look at this using Vic's stuff at TOI and your info at BtN.
Scott, I used defensemen with TOI>22 and 20-70 GP:

It's 10-15 guys per season.
Thanks for the link... it was a question motivated mostly by laziness :)
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