Friday, November 1, 2013

On Corsi Rates and Corsi +/-

Occasionally I feel the need to look for old data, back when Corsi and Fenwick were new and when was updated at the beginning of every season, without exception.  That brings me to the old version of, which expressed our favorite numbers as a whole number, not as a rate.  Jack Johnson wasn't at 43.9% in 2008-09 according to timeonice, he was at -131 - that was his Corsi +/-.  This quickly became non-standard, and so old timeonice numbers became a pain - you had to do tedious calculations to get at territorial percentage numbers, which is all everyone seems to care about now.  Still, I miss the whole numbers for three reasons:

A:  You can approximate a player's expected +/- based on a 1000 PDO.  I don't have average Fenwick and Corsi shooting percentages handy - I know they're floating around somewhere - but we know that on-ice even strength shooting percentages are around 8%, typically.  So if Jack Johnson is -81 in shots in 2008-09, we know that goals are scored on around one out of every 12.5 even strength shots, so he should be approximately -6 with average goaltending and average shooting.  Henrik Zetterberg was +332 shots in 2007-08, meaning he 'should've been' +26 at 5 on 5 hockey.  With rates, there's no way to estimate this except 'higher = good, lower = bad'.

B:  High-event players get privileged over low-event players, which may be correct - Imagine a thought experiment - one player is on the ice for 90 shots/60 for his team and 60 shots/60 against.  Another is on the ice for 30 shots/60 and 20 shots against.  They have the same shots rate - 60% - but Player A is going to be putting up +30 shots/60 whereas Player B will only be doing +10.  Indeed, we can find real world examples of this - Jonathan Cheechoo and Alexei Ponikarovsky played about the same amount of ice time in 2007-08.  They put up similar shot %s - 57.5% for Ponikarovsky, 56.7% for Cheechoo.  Cheechoo even had a little more ice time than Ponikarovsky.  And yet, Ponikarovsky is +121 in Shots, Cheechoo +104, because Ponikarovsky's lines shot the puck more than Cheechoo's but also gave up more shots.  It's not much, but Ponikarovsky should've had an extra goal (and a half).  When we go by rates, we miss out on the extra value that higher event players may provide*.

C:  It's closer to a WAR-type thing.  Yes, we have to adjust for Zone Start.  Yes, we have to adjust for Quality of Teammates.  We may even have to adjust for Quality of Competition.  But we have a better sense of how many goals this player is adding to or subtracting from their team.  4th line players get dinged for having fewer total events.  Injured players get dinged for having been injured.  One and two game callups no longer look like superstars or complete disasters - they just look like exactly what they are: guys who might've had a good night, bad night, or an inbetween one.

I know that territorial metrics can't come close to approximating a player's actual value, but when we go by rates, we're missing a larger element - ultimately, it's not rates that win games, but shots.  We should be talking about rates for teams and more about whole numbers for individual players.

* - High event players may not be better than low event players because of the NHL's point structure - the OT point implicitly rewards low event teams, as games with fewer events should have fewer goals and games with fewer goals are more likely to be tied at the end of 60 minutes.  Still, were this not the case, high event players with the same shot % would be better than low event players.

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