Last week Tyler Dellow had a post titled “Two Graphs and 480 Words That Will Convince You On Corsi%” in which, you can say, I was less than convinced (read the comments). This post is my rebuttal that will attempt to convince you on the importance of Sh% in player evaluation.
The problem with shooting percentage is that it suffers from small sample size issues. Over small sample sizes it often gets dominated by randomness (I prefer the term randomness to luck) but the question I have always had is, if we remove randomness from the equation, how important of a skill is shooting percentage? To attempt to answer this I will look at the variance in on-ice shooting percentages among forwards as we increase the sample size from a single season (minimum 500 minutes ice time) to 6 seasons (minimum 3000 minutes ice time). As the sample size increases we would expect the variance due to randomness to decrease. This means, when the observed variance stops decreasing (or significantly slows the rate of decrease) as sample size increases we know we are approaching the point where any variance is actually variance in true talent and not small sample size randomness. So, without going on any further I present you my first chart of on-ice shooting percentages for forwards in 5v5 situations.
Variance decline pretty much stops by the time you reach 5 years/2500+ minutes worth of data but after 3 years (1500+ minutes) the drop off rate falls off significantly. It is also worth noting that some of the drop off over longer periods of time is due to age progression/regression and not due to reduction in randomness.
What is the significance of all of this? Well, at 5 years a 90th percentile player would have 45% more goals given an equal number of shots as a 10th percentile player. A player one standard deviation above average will have 33% more goals for given an equal number of shots as a player one standard deviation below average.
Now, let’s compare this to the same chart for CF/20 to get an idea of how shot generation varies across players.
It’s a little interesting that the top players show no regression over time but the bottom line players do. This may be because terrible shot generating players don’t stick around long enough. More importantly though is the magnitude of the difference between the top players and the bottom players. Well, a 90th percentile CF20 player produces about 25% more shots attempts than a 10th percentile player and a one standard deviation above average CF20 player produces about 18.5% more than a one standard deviation below average CF20 player (over 5 years). Both of these are well below (almost half of) the 45% and 33% we saw for shooting percentage.
I hear a lot of ‘I told you so’ from the pro-corsi crowd in regards to the Leafs and their losing streak and yes, their percentages have regress this season but I think it is worth noting that the Leafs are still an example of a team where CF% is not a good indicator of performance. The Leafs 5v5close CF% is 42.5% but their 5v5close GF% is 47.6%. The idea that CF% and GF% are “tightly intertwined” as Tyler Dellow wrote is not supported by the Maple Leafs this season despite the fact that the Maple Leafs are the latest “pro-Corsi” crowds favourite “I told you so” team.
There is also some evidence that the Leafs have been “unlucky” this year. Their 5v5close shooting percentages over the past 3 seasons have been 8.82 (2nd), 8.59(4th), 10.54(1st) while this year it has dropped to 8.17 (8th). Now the question is how much of that is luck and how much is the loss of Grabovski and MacArthur and the addition of Clarkson (who is a generally poor on-ice Sh% player) but the Leafs Sh% is well below the past few seasons and some of that may be bad luck (and notably, not “regression” from years of “good luck”).
In summary, generating shots matter, but capitalizing on them matters as much or more.