ESPN's GameCast feature shows shot charts for every NHL game, which is quite useful to indicate the territorial play and to help evaluate goalie performance. I am a big fan of shot quality metrics like the one developed by Alan Ryder, but one of the limitations is that it reduces a team's shots against distribution to a single number. Even if we know that a team's shot quality against is 0.95, we don't know whether that is because they allow a lot of perimeter shots, or whether they are good at preventing in-close chances. I think this might be one of the reasons why some fans seem hesitant to trust the shot quality numbers.
As a result, I have developed my own system, using my typical brute-force approach. The objective is to achieve the dual goal of approximating scoring probabilities while also being able to qualitatively describe the type of chances a team gives up. Based on the scoring probability information given in this blog post from Behind the Net, I divided the rink up into 5 zones: Crease Area, Slot, Mid-Range, Point, and Perimeter. I used rink markings to divide the separate zones, which isn't exactly correct based on scoring percentages but becomes somewhat necessary for classifying chances. For the sake of time, everything is eyeballed except for chances from the crease area, which are defined as anything within 15 feet of the net inside the edges of the crease.
Here is the rink diagram:
The primary drawback of this technique is that it does not differentiate between power play and even-strength chances. I could split them out, but obviously it takes a bit of time to go through and count the chances so for now I'm lumping them all together. There are also certain rinks that seem to measure shot distances differently than everyone else (see this article by Alan Ryder), which means that it is not really fair to compare results directly between, say, goalies playing for the New York Rangers and goalies playing for the Tampa Bay Lightning. I'll have to work on some way to adjust for these discrepancies.
Just to be clear, I am not presenting this as an improvement on existing shot quality metrics. I hope the results will be similar, but I would certainly defer to other methods if there is a disagreement because they are much more precise in terms of identifying exact shot distance, shot type, etc. The primary reason for doing this is to get a better sense of the type of shots each goalie is facing. Is he facing a lot of shots from the perimeter? How well does his team cover the point shot? How many close-in chances does he face? And so on.
I don't have a large enough sample size to get high-confidence estimates of scoring likelihood from each area, but based on a sample from last year's playoffs the approximate average save percentages are:
Crease Area: .800
I'm planning to use this method to break down this year's playoff results, as well as to take a closer look at Martin Brodeur vs. his teammates in my continuing look at how goalies contribute to shots against. Criticisms and suggestions are welcome in the comments or via email.