by PuckStopsHere on 07/18/09 at 06:36 AM ET
In this summer’s look at sabermetrics and hockey, I have been looking at the Corsi Number as an alternative to +/- ratings. Its appeal is that it includes a lot more events than +/- (as it is the difference between shots directed at the goal for and against when a player is on the ice in 5 on 5 situations). The drawback is that it includes missed shots and blocked shots as being equivalent to goals. Any systematic reason why a player or team might take or allow an excessive number of shots that miss that net (or some other anomaly like that) would skew the ratings. Yesterday, I gave the team Corsi Numbers from last season and it led to a very good discussion. Some teams have Corsi Numbers that do not correlate well with their regular season success. For the most part this can be explained (the comments to that post contain some of this discussion). For example, Boston has the 13th best team Corsi Number at +49 despite finishing first overall
This shows that Boston did not carry the play so well in even strength situations. They allowed almost as many shots as they took. They did well because of outstanding goaltending from Vezina winner Tim Thomas and from Manny Fernandez. In order for Boston to repeat at that level, it is necessary for them to get equally strong goaltending from Thomas and Tuukka Rask this season or to see further improvement from their young core of players.
Another team with a low Corsi Number is the Philadelphia Flyers. Their 24th place -347 Corsi is the worst of all playoff teams in 2008/09. This seems odd given that they had a team +/- of +24 which placed them sixth in the league. This is partially explained when looking at the difference between the situations where +/- and Corsi apply. Corsi is only calculated in 5 on 5 situations, while +/- is calculated to include even strength and short handed goals. Philadelphia led the NHL with 16 short handed goals last year and allowed only one. Their +/- is fifteen points higher due to short handed scoring. Taking that away, Philadelphia falls significantly in the +/- standings (though not as far as their Corsi would suggest). Philadelphia is a team that well on the power play for the last several years. That makes up for some more lacklustre play at even strength and keeps them doing relatively well in the standings. Power play success shows up in neither +/- nor Corsi.
Do Corsi and +/- more or less measure the same thing? The team values for each can be plotted against each other to give the plot below.
It is clear that in general teams with good +/- ratings also have good Corsi Numbers and vice versa. The correlation between Corsi and +/- in 2008/09 was 0.462. This is a pretty good correlation. They are measuring similar features of hockey.
There are seven teams where the Corsi and +/- have differing signs. Four have positive +/- and negative Corsi. They are Vancouver, Florida, Pittsburgh and Philadelphia. The Philadelphia case is largely explained by shorthanded play, as explained already. The Vancouver case is one of strong goaltending. Roberto Luongo saved a higher percentage of shots taken on him than did Vancouver’s opponents. Thus the shots taken favored the opponents, while the goals favored Vancouver. Florida was a similar case with Tomas Vokoun and Craig Anderson providing top goaltending. Pittsburgh did very badly in the early part of the season and their Corsi never recovered in their late season turn around. Their top forwards all had very good shooting percentages, which imply that they tended to take higher quality shots and may have chosen not to take the lesser quality ones. There are three teams with positive Corsi and negative +/-. They are the New York Rangers, Los Angeles and Toronto. Toronto is the reverse of the Vancouver case. They had very bad goaltending. They a relatively few number of shots, but they went in the goal. Los Angeles is another case of relatively poor goaltending (though not as extreme). The New York Rangers were a case of very bad shooting percentages. They took a lot of shots but few went in. In fact, the top shooting percentage among anyone who played 60 or more games with the New York Rangers was held by Markus Naslund at 11.2%. That would be ninth highest on Pittsburgh (tenth if you include Chris Kunitz who was acquired just before the trade deadline).
Corsi Numbers vary from +/- largely for three reasons. The inclusion of shorthanded goals in +/- can change things significantly in a few cases. Teams with goaltending that is worse than or better than average will see different results in their shots allowed and goals allowed and similarly teams with better than or worse than average shooting percentages will see the same results, but in reverse.
The idea behind looking at +/- ratings and Corsi Numbers comes from an attempt to follow baseball sabermetrics. It is well known in baseball that runs scored and allowed is a better predictor of future success than win loss records. Similarly, it is true that goals scored and allowed (which is essentially the idea of +/-) better predict future success than win loss records. Baseball can be broken down into sub-elements of runs scored (hits, walks, stolen bases etc. for and against) and this is a better predictor still of future success than runs for and against. It is hoped that something can be found in hockey to correlate with that. Corsi is a first attempt to find a sub-element of goals scored. It isn’t successful in being a better predictor of success than +/-, but it may be a step in the right direction.
Hockey makes the distinctions between special teams and even strength play in these numbers (where baseball has nothing similar) to try to compare between teams and players in similar situations. If we are looking at trying to predict the future success of teams, these distinctions are not so important, but for now they remain in my data. We can look at how team Corsi and +/- ratings correlate with the point totals that teams get. First here is the plot of +/- versus points:
And here is a plot of Corsi versus points.
Team +/- ratings have a very strong correlation with points of 0.880. Team Corsi Numbers have a strong (but lower) correlation of 0.626. A significant part of the better correlation from +/- comes from the fact that it includes shorthanded goals and Corsi does not. What is interesting is that both Corsi and +/- ratings correlate better with points than they do with each other. This shows that they are somewhat complimentary measures of team success. It is usually true that if a team has a high Corsi and low +/- (or vice versa) the results of the team are somewhere in the middle of both.
Corsi Numbers are useful to show team success. They do not do as good a job as +/- ratings, but they offer some complimentary information that +/- does not. This shows a significant value in studying Corsi Numbers. Some trends, such as Boston’s failure to carry the play at even strength, are obvious when looking at Corsi Numbers but obscured by +/-. This is one goal of sabermetrics. To be able to document trends in a sport that were previously outside of direct measure. In some occasions, Corsi can allow this.
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