Dec 152010
 

I have been pretty quiet here recently not because of a lack of things I want to write about but because I needed to get my stats site up and running first so I can reference it in my writings.  Plus, getting my stats site up has been on my todo list for a real long time.  There will be a lot more stats to come including my with/against on ice pairing stats which I had up a season or two ago and many of you found interesting as well as team stats but for now let me explain what is there.

What you will find there now is my player rating system which produces the following ratings:

HARD – Hockey Analysis Rating – Defense

HARO – Hockey Analysis Rating – Offense

HART – Hockey Analysis Rating – Total

HARD+ – Hockey Analysis Rating – Defense

HARO+ – Hockey Analysis Rating – Offense

HART+ – Hockey Analysis Rating – Total

HARD is the defensive rating and is calculated by taking expected goals against while on the ice and dividing it by actual goals against while on the ice.  The expected goals against is calculated by taking the average of a players team mates goals against per 20 minutes (TMGA20) and averaging it with the players opposition goals for per 20 minutes (OppGF20).  Similarly HARO is calculated by taking a players actual goals for while on the ice and dividing it by the expected goals against while on the ice.  For both, a rating above 1.00 means that the player helped the team perform better than expected when he was on the ice where as a rating below 1.00 means the player hurt the teams performance when he was on the ice.  HART is just an average of HARD and HARO.

HARD+, HARO+ and HART+ are enhanced ratings which result from an iterative process that iteratively feeds HARD and HARO ratings into an algorithm to refine the ratings.  For the most part this iterative process produced a nice stable state but sometimes the algorithm goes haywire and things fail (i.e. for a particular season or seasons).  For this reason I am calling the + ratings experimental but if you don’t see anything wacky (i.e. large differences in every players ratings) they should be considered reliable and probably better ratings than the straight HARD, HARO and HART ratings.  Anything better than 1.00 should be considered better than the average player and anything less than 1.00 should be considered below average.

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Nov 222010
 

There are two things that must occur to score a goal.  The first way is to get an opportunity to score and the second is to capitalize on that opportunity to score.  There are a number of statistics that we can use as a proxy for opportunity to score but one of the most common is Fenwick numbers which are shots + missed shots (some call this Corsi but I define Corsi as shots + missed shots + blocked shots).  We can then define the ability to cash in on opportunities as shooting percentage, or in this case fenwick shooting percentage.  So let me define the following:

Opportunity Generation = Fenwick shots per 20 minutes of ice time.

Capitalization Ability = Fenwick Shooting Percentage = Goals Scored / Fenwick shots

So the question I pose today is this:  What is more important in scoring goals, generating opportunities or the ability to capitalize on those opportunities.  To answer this I calculated each teams Fenwick per 20 minutes (opportunity generation) and each teams Fenwick Shooting Percentage (capitalization ability) and compared them to the number of goals they generated per 20 minutes of ice time and I did this for each of the past three seasons (I only considered even strength five on five data).  I also did this for both the offensive and defensive ends of the ice for a total of 90 data points offensively and defensively.

First for the offensive end of the game:

As you can see, shooting percentage (opportunity capitalization) has a much stronger relationship with scoring goals than getting shots (opportunity generation).  What about the defensive end of the game?

Again, opposition capitalization rates are much more correlated with scoring goals than opportunity generation.  In fact opportunity generation appears to have no correlation with giving up goals at.

The conclusion we can draw from these four charts is when it comes to scoring goals, having the ability to capitalize on opportunities (shots) is far more important than having the ability to generate opportunities (getting shots).  Controlling the play and generating shots does not mean you’ll score goals (just ask any Maple Leaf fan), having the talent to capitalize on those opportunities is what matters most.  From my perspective, this means the usefulness of ‘Corsi Analysis’ to be minimal, at least for the purpose of evaluating players and teams.  For evaluating goaltender workload, as it was initially intended by its originator former NHL goalie and Buffalo goalie coach Jim Corsi, it still has merit.

Jun 092010
 

Behind the Net Blog recently used even strength when game is tied Corsi analysis to take a look at the divisional imbalance since the lockout and came up with an interesting conclusion.

The NW division is slightly better than the SE division against all shared opponents.  But SE division teams outshot NW teams in head-to-head games.  The difference between the two divisions is negligible, though the NW’s stronger showing against the pacific and central suggests that it’s just a little bit better than the SE.

What that essentially implies is that since the lockout the northwest division is only marginally better than the southeast division, which has generally been considered the worst division in hockey since the lockout.

This though is a perfect example of where Corsi analysis fails because that statement is proven downright untrue when you consider each divisions actual won-loss records.  Against the southeast division the northwest has combined for a dominating 64-31-12 and only twice has a northwest team had a losing record against the southeast (2009-10 Wild at 1-2-3 and 2008-09 Flames at 1-3-1).  The 64-31-12 record is the equivalent to a 107 point team over 82 games which is awfully good.  The southeasts record against the northwest is 43-49-15 which is equivalent to a 77 point team.  To put that in perspective, the NW is like Phoenix (107 points) and the SE is like Columbus (79 points) this past season.  That makes the northwest division more than ‘a little bit better’ than the southeast division.

In another analysis at Behind the Net blog they look at Corsi +/- for teams in games against teams in divisions other than their own.  For the northeast division they came up with:

Ottawa +200, Boston +134, Toronto +65, Buffalo -60, Montreal -266.

That would seem to indicate that Toronto has been a halfway decent team but they finished last in the northwest division in 3 of the 5 seasons and never finished better than 3rd.  Montreal finished ahead of the Leafs in 4 of the 5 seasons and accumulated 49 additional points in the standings despite having an outside division even strength when game is tied Corsi +/- a whopping 331 points below that of the Leafs.    The Minnesota Wild had a very dismal -419 Corsi +/- outside the division but had a respectable 134-106-26 record which is equivalent to a 91 point team.  Now a 91 point team is nothing special, but it is a far cry from what the 3rd worst outside division Corsi +/- would indicate their record ought to be.

In both of these posts the use of Corsi analysis has failed to accurately explain what really happened on the ice and it comes down to the fact that even strength when the game is tied Corsi numbers only tell a fraction of the story.  It doesn’t account for goaltending or power play or penalty kill or shooting ability or any number of other factors that influence who wins hockey games so using it as a tool for determining which teams or divisions are better is a pointless exercise because on the ice, all those other things matter.  The better tool to use in evaluating which teams or divisions are better is the much simpler and more universally understood statistic known as win-loss records.  Win-loss aren’t perfect, but they don’t try to tell me that the Leafs have been better than Montreal since the lockout or that the northwest division is only marginally better than the southeast division.

Jun 032010
 

I am planning that over the course of the summer and into next season I will get back into analyzing hockey statistics more in depth again.  Over the past couple of seasons Corsi numbers have become much more prevalent so I thought I would start off by discussing what they are and my thoughts on them.

Corsi numbers were originally created by former NHL goalie and now Buffalo Sabre goalie coach Jim Corsi.  David Staples recently had a good interview with Corsi which goes into his thought process behind developing Corsi numbers.  The interview is definitely worth a read but let me summarize.

In his role as the Sabre’s goalie coach, Corsi was attempting to evaluate the work load his goalies had in a game of play and found that simply shots against were not sufficient.  The goalie can relax whenever the puck is in the oppositions end, but whenever the play is in his own end he can’t relax, regardless of whether a shot was taken or not.  To get a better idea of his goalies workload he summed up shots, missed shots and blocked shots which should give a much better indication of a goalies overall work load.  A goalie needs a certain skill level to successfully save the majority of shots on goal, but a goalie also needs a certain fitness level (both mental and physical) to be able to play under a certain workload level within a single game and over the course of an 82 game season and this is why Corsi invented the Corsi numbers.

More recently others in the hockey community have extended Corsi numbers to evaluate a teams ability to control the play of a game (i.e. does a team play more in the oppositions zone vs their own) and evaluate individual players by looking at their Corsi numbers for and against while they are on the ice and comparing that to their teammates Corsi numbers.  Most notable are Gabe Desjardins of behindthenet.ca and Gabe and everyone else at the Behind the Net blog but there are others too.  Some people, most notably Matt Fenwick of the Battle of Alberta blog only use shots and missed shots and do not include blocked shots as Jim Corsi does resulting in what is typically called Fenwick numbers.  When used in this context Corsi and Fenwick numbers are calculated just as +/- is calculated which is to take the shots+missed shots+ blocked shots for his team and subtracting the shots+missed shots+ blocked shots numbers by the opposition while he is on the ice.

One of the benefits that many people believe that Corsi numbers provide is that since Corsi numbers include more events (i.e. shots+missed shots+blocked shots vs just shots or even just goals as in +/-) the statistical analysis will be far more accurate due to the larger ‘sample size.’

So what do I think of all this?  I do agree with Jim Corsi that using Corsi numbers as a way to evaluate a goalies workload is probably far more valuable than just using shots on goal.  Beyond that, I am pretty sure that Corsi numbers will give a pretty solid indication of a teams control of the play, for whatever that is worth.  I say for whatever that is worth because some teams, when they have the lead, will choose to play in a defensive shell allowing a lot of shots from the point, but not giving up all that many high quality, in close, shots or worse yet, shots on rebounds. Corsi numbers when the game is close (tied, or within one goal with significant time to play such that the team with the lead has not yet gone into ‘protect the lead’ mode) may give us a better indication of a teams capability to control the play, when they want to but even that may be flawed.  Also, a team with a strong set of forwards but a weak defense and goalie may control the play more than a team with a strong defense and top tier goalie but is that team really any better at winning games?

Much of the same arguments can be made when evaluating players.  Defensive minded players are not necessarily on the ice to control the play, they are on the ice to not allow goals against most typically by the oppositions top offensive forwards.  As mentioned above, one way to accomplish this is to go into a defensive shell and just not give up any quality scoring chances against.  A player can have a sub-par Corsi number, but be doing his job perfectly well.

I do believe that Corsi numbers have a use in evaluating a goalies work load and even in showing which teams are controlling the play, but in my opinion using it anywhere beyond that we are making too many assumptions about how important Corsi numbers are with respect to winning games.  Just ask the Washington Capitals how almost completely controlling the play worked for them against Montreal in round two of the playoffs. In the past I have used mostly goals for/against and shot quality (using shot type and distance as a proxy for quality) to evaluate players and while that has its own inherent flaws as well I will most likely continue to do so in the future.