Friday, November 19, 2010

Adjusted Corsi w/ Score Close - Part III

In two previous posts, I showed how each team had performed in terms of Corsi percentage with the score close at that particular point in the season, and then adjusted each team's percentage in order to account for variance in strength of schedule.

While my strength of schedule adjustment corrected for game location (i.e. whether, from a particular team's perspective, the game was being played at home or on the road), it did not account for back-to-games. As it turns out, this was a mistake on my part -- the effect of back-to-games on Corsi (with the score close) is much larger than the effect of game location.

Table Legend
GP = Games played; The number of games played in the 2010-11 season which meet the criterion in the left hand column
R Corsi = Road Corsi; The number of shots directed at the net at even strength by the road team
H Corsi = Home Corsi; The number of shots directed at the net at even strength by the home team
R Corsi % = Road Corsi Percentage; the Corsi percentage from the road team's vantage

The above table shows how game location and whether or not the game is a back to back for either team have interacted to effect Corsi percentage with the score close in games played during the 2010-11 season thus far (up to and including the 268th game).

Some observations:

Firstly, in games in which one team played the night before and the other team did not, the road team carries the burden roughly 75% of the time. I guess the league wants to give the home team every advantage it can without making it seem too obvious.

Secondly, the effect of back-to-back games on Corsi percentage with the score close is considerable -- approximately 0.04. In other words, an average team playing a likewise average team that played the previous night can expect to achieve Corsi results on par with those of the 2009-10 Boston Bruins (a strong, if underrated, team). If the positions are reversed, however, it can expect to perform more along the lines of the 2009-10 Tampa Bay Lightning. That's a sizable difference.

Thirdly, the effect of game location, in and of itself, is pretty small. In games in which both teams are playing on at least one day's rest, the home team only marginally outshoots its counterpart.

With these findings in mind, I decided to modify my method of correcting for scheduling difficulty by accounting for the effect of back-to-back games in addition to game location. Here are the results for all games played as November 17.

More observations:
  • The Capitals raw numbers are ok, but they really get killed by the schedule adjustment. I think that they've played the Thrashers three times already
  • Chicago's numbers might come across as underwhelming, given where they were last year, but they're improved in this regard as the season has progressed. I expect that to continue
  • The Oilers are a real outlier. They're 0.095 from the mean when the next furthest team is a mere 0.064. Things don't look good
  • The Wings may not be what they once were in terms of territorial dominance, but they've still managed to best everyone else. It's hard not to have some degree of admiration for that club


Sunny Mehta said...
This comment has been removed by the author.
RyanV said...

It's funny, because I was literally just looking into this last night and concluded the exact opposite: that location mattered more than back-to-back. I looked at wins/losses and goal differential rather than corsi, but I also looked at every game since 2007/08.

For what it's worth, the win/loss data makes it look like this year so far is an outlier. Road teams, on non-b2bs, this year have won 47.7% compared to 45.7% over the last three-and-a-quarter seasons. And road teams on b2bs have won only 38.2% compared to 42.5% since 07/08.

I'll check into it with corsi and see what I find.

RyanV said...

So here's what I get since 07/08. This is excluding the four games that are missing play-by-plays. "Home" means _only_ the home team had played the night (or afternoon) before, and similarly for "road". "Both" means that both teams played the night before. The percentage is the percentage of wins/goals/5-on-5 corsi in favour of the home team. (I'm counting shootout wins as wins and winning a shootout counts as a goal for. This shouldn't change the results over so much data.)

Win% Goal% Corsi%
Neither: 54.35% 52.21% 51.21%
Home: 54.29% 52.15% 50.15%
Road: 57.99% 54.62% 52.47%
Both: 55.77% 54.08% 52.13%

Looks to me like playing back to back is nearly exactly as big a disadvantage in terms of 5-on-5 corsi as playing on the road without a back to back.

JLikens said...

Thanks for running the numbers, Ryan.

I suppose this serves as a lesson to never rely on 1/5 of a season as a sample when attempting to draw such conclusions.

In any event, your results make much more sense -- we'd expect the home team to have some 'inherent' advantage with respect to outshooting, even if that advantage is exaggerated by scheduling factors.

pitseleh said...

Interesting to see that Vancouver manages to finish fourth despite having the worst puck possession line in the league as their fourth line. I guess their limited ice time doesn't hurt them that much in the grand scheme of things.

Thanks for the update.

JLikens said...

Yeah, the Canucks 4th line forwards have been substantially underwater in terms of Corsi for several years running now.

The fact that Vigneault prefers to start them off in their own end does nothing to help them in this regard.

(I suppose it's more of an open question as to whether such a strategy benefits the team as a whole).

Kent W. said...

(I suppose it's more of an open question as to whether such a strategy benefits the team as a whole).

It probably does. It would help if they were better players on the 4th, but burying the pluggers puts guys like the Sedins in really nice circumstances and they are good enough to make other teams pay for it.

Lortimer said...

You posted over on Behind the net that corsi strength of schedule for Colorado is not meaningful over a season.
Am I missing something? If not, why go to the trouble to adjust the totals in this post?
Just wondering?

JLikens said...

Two points:

1. The term "meaningful" is context sensitive. In my post at behindthenet, I meant to say that the magnitude of the schedule difficulty adjustment for the average team isn't significant enough to account for the discrepancy between the predicted model and actual results.

For the purposes of team evaluation, however, the adjustment is more meaningful.

2. The league wide variation in strength of schedule decreases as the season progresses. The fact that correcting for schedule difficulty at the season's end may not be worthwhile doesn't necessarily mean that doing so at this point in the year is pointless.

Host Pay Per Head said...

I didn't expect that there is a variance in the strength of the schedule but at least it proves that it is something that it makes a difference.