Sunday, March 29, 2009

The Blackhawks

The Blackhawks have been an interesting team this year.

First of all, there's plenty of evidence which suggests that the Blackhawks are a pretty solid team. They've accrued one of the best goal differentials in the league up to this point in the season, despite playing in the league's toughest division in the better of the two conferences. They've also soundly outshot the opposition, both at even strength and in general. In these respects, it would be difficult to argue that the Blackhawks are not one of the league's best teams.

What I find unusual, then, is that the Blackhawks expected goal differential, as calculated at hockeynumbers, is only slightly positive. The Blackhawks have allowed some 20 fewer goals than what would be predicted on the basis of shot quality, while having scored some 20 more. Given that their shot ratio is more or less in line with their goal ratio, the implication is that Chicago has been below average in both shot quality for and shot quality against. Indeed, if the expected goals numbers are translated to yield a shot quality index for each team, the Blackhawks do in fact fare quite poorly.

The fact that the Blackhawks tend to allow high quality shots against is not surprising. Firstly, shot quality is repeatable on a year-to-year basis, with Chicago having ranked 27th in the league in that regard last year.

Secondly, teams coached by Joel Quenneville tend to allow high quality shots against on average. Outside of 1999-00, for every year that Quenneville has been a head coach in the NHL, his team has ranked in the bottom half of the league in terms of save percentage, which implies that his teams were surrendering high quality shots against. While it's perhaps true that Quenneville was burdened with poorer than average goaltending during his tenure in both St. Louis and Colorado, shot quality has been directly measured from the 2002-03 season onward, with the results tending to support the argument that Quenneville-coached teams are poor in terms of shot quality against.

The Contrarian Goaltender has also found evidence that the effect of Quenneville's coaching in St.Louis was to reduce the save percentages of his own goalies, who tended to have better save percentages prior to playing under Quenneville (see, for example, the comments section of this post).

With a team save percentage of 0.912, the Blackhawks goaltending has been nothing short of superb this season. However, the fact that the team apparently allows such high quality shots against, not to mention the fact that save percentage is at least partially a product of random statistical variation, inevitably leads to the question of sustainability. Of course, It's true that both Khabibulin and Huet have proven track records, and both goaltenders are certainly better than anything Quenneville had to contend with in St. Louis and Colorado. Nonetheless, I think that it's a potential cause for concern and, at the very least, something that one ought to be mindful of in evaluating the team's prospects for the postseason.

As for the team in general, I'm inclined to think that they're still pretty good, the shot quality numbers notwithstanding. As I intimated in the above paragraph, both Huet and Khabibulin are above average netminders and the Blackhawks team save percentage is in that sense somewhat sustainable. And while it is true that the Blackhawks have exceeded their expected goals for, I'm reluctant to ascribe the difference to luck considering their plethora of offensively talented and creative players. Furthermore, even if I happen to be wrong on these points, their tendency to convincingly outshoot the other team is, if nothing else, encouraging.

7 comments:

sunnymehta.com said...

You know, one effect that I don't hear mentioned wrt teams outperforming the SQ model offensively is the possibility of facing bad goaltending.

I remember emailing Alan Ryder about why (in his PC ratings) he uses his SQ model and adjusts for goaltending when evaluating individual players' defensive PC points, but not when evaluating their offensive PC points (he essentially uses raw goals). His response was that he's making the assumption that a position player faces league average goaltending throughout a season but doesn't necessarily have league average goaltending behind him.

That explanation doesn't jive with me for some reason. It seems like there could be huge differences in the goaltending that players face in the sample of one season. Further, either the SQ model works or it doesn't, right? If it works, why just use it for defense? From what I've read on the subject, there appears to be no real evidence that offensively gifted players outperform the model with any persistence.

Getting back to the Blackhawks, I have no idea if facing weak goaltending is the culprit. However, I did notice that both Detroit and St Louis both outperformed their expected GF, and neither Columbus or Nashville underperformed by much. I bring up those four teams because they're all in the same division and presumably play similar schedules.

JLikens said...

The idea that outperforming expected goals at the team level could be a product of facing weaker-than-average goaltending is an interesting one.

In theory, there's no good reason why there wouldn't be such an effect. However, the impact would necessarily be small, given that the quality of oppositional goaltending would not, I presume, vary widely across teams.

As for the Blackhawks, I ran the numbers in order to determine the extent to which they've benefited from facing weak goaltending.

I calculated the average (weighted) save percentage of Chicago's opponents this season, removed the impact of empty netters, removed the effects of the Hawks themselves on the data, and came up with 0.9076, which is only very slightly lower than the league average of 0.908.

I also calculated the shot quality neutral save percentage of Chicago's opponents, also removing Chicago's effects upon those numbers, and came up with 0.9066 (league average SQN SV %: 0.908).

So while it does appear that Chicago has faced weaker than average goaltending, the effect is not large.

sunnymehta.com said...

Thanks for running the numbers, J. But isn't it kind of a horse-and-carriage thing? In other words, if you did the same thing for the shooting percentage faced by Chicago's goaltenders, you'd probably get something close to league average, right?

It just seems strange to me that if a shooter's outperformance of a SQ model is usually due to randomness, a goaltender's outperformance isn't also heavily affected by randomness. People seem quick to give credit to a goalie who posts a good SQNSV%, but really has research been done on persistence on that end? My selective memory tells me that goalies seem to vary quite a bit from year to year in their SQNSV%.

JLikens said...

"Thanks for running the numbers, J. But isn't it kind of a horse-and-carriage thing? In other words, if you did the same thing for the shooting percentage faced by Chicago's goaltenders, you'd probably get something close to league average, right?"

I'm sorry, but I'm not sure that I understand the point being made here.

Yes, I suspect that the shooting percentage (or SQN shooting percentage, if you will) of Chicago's opponents this year would be close to the league mean. However, that would merely indicate that Chicago's high team SQN SV % cannot be explained by the tendency of their opponents to underperform relative to expected goals.

I suspect that there might be a flaw in my logic here, but I'm not sure what it is. What have I overlooked?

"It just seems strange to me that if a shooter's outperformance of a SQ model is usually due to randomness, a goaltender's outperformance isn't also heavily affected by randomness. People seem quick to give credit to a goalie who posts a good SQNSV%, but really has research been done on persistence on that end? My selective memory tells me that goalies seem to vary quite a bit from year to year in their SQNSV%."

A valid point.

I've done a bit of research in this area and I can tell you that you're absolutely correct in that SQN SV % is heavily subject to randomness.

There is a (positive) year-to-year correlation among goaltenders in terms of SQN SV %, but the correlation is not large and is similar in magnitude to raw save percentage. The strength of the correlation varies as a function of sample range (for example, by including marginal NHL goaltenders in the sample, this increases the sample range and strengthens the correlation accordingly).

However, it is also true that SQN SV% is more reliable than SQN shooting percentage, if I may coin such a term. That is to say, a team's tendency to over or under perform its expected goals against is more repeatable than the tendency to over or under perform expected goals for.

Some inter-year correlations (data corrected for arena bias):

SQN Save Percentage

06-07 -- 07-08: 0.33
05-06 -- 06-07: 0.46
03-04 -- 05-06: 0.35
02-03 -- 03-04: 0.51

SQN Shooting Percentage:

05-06 -- 06-07: 0.15
03-04 -- 05-06: 0.16

There's no data on shot quality for at the team level for 02-03 or 07-08.

Therefore, while both are mildly reliable, the inter-year correlations are stronger for SQN Save percentage.

sunnymehta.com said...

Hm that last part is interesting, and it gets to the crux of what i was questioning. So SQNSV% is more reliable than SQS%. I'll have to ponder that.

Btw, are these the '08 numbers you're looking for?...

http://hockeystats.no-ip.org:81/nhl08/nhl.php?id=8

JLikens said...

"Btw, are these the '08 numbers you're looking for?..."

Thanks.

I was wondering if Chris had archived the data for previous years.

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"I take short fucking showers man" PK