David Hutchison | Jan 22, 2019 | 0
An Introduction To Adjusted Save Percentage
As Paul Campbell pointed out in a recent article here on InGoal, it is extremely difficult to quantify a goaltender’s statistical value to a hockey team. Difficult, but not impossible.
Many analysts suffer because they try to understand goaltenders with outdated statistics, or they simply have a lack of knowledge about how the position is played.
The truth is, traditional statistical evaluations of goaltenders have been extremely flawed since the day the game began. Goals-against-average and wins were the first major categories in the early years. In reality, goaltenders have very little control over those numbers. It’s a bitter pill to swallow for many people that grew up idolizing those traditional stats, but they offer little-to-no insight about an individual goaltender’s performance.
Eventually, sometime in the 80s, save percentage rose to the forefront and became the most common goaltending statistic. Analysts were finally using a stat that actually reflected a goaltender’s performence. Unfortunately, research has proven that it is also flawed. With traditional “unadjusted” save percentage, every shot is weighted equally. A dump-in from the neutral zone that reaches the goal is worth the same amount as a one-timer from the slot. Wouldn’t it be great to fix that?
Welcome to the world of adjusted save percentage.
Why is Adjusted Save Percentage better?
Adjusted save percentage is a step up from traditional save percentage because it takes shot location into account. Since goalies have almost no control over where the shots they face come from, they are no longer unfairly punished because they play on a team that gives up more shots closer to the net. Alternatively, a goaltender that faces more shots from a distance does not get rewarded.
While shot location does not tell the entire story, shots from each location on the ice do come with some level of predictability. War-on-Ice has defined three different zones, each with different levels of expected shooting percentage.
In the blue or “high-danger” zones, each shot has a 10%+ chance of going into the net.
In the red or “medium-danger” zones, each shot has a 3-10% chance of going into the net.
In the yellow or “low-danger” zones, each shot has a 3% or below chance of going into the net.
It is a simplistic, but effective proxy for shot quality, in the same way that “Corsi” and “Fenwick” are proxies for puck possession. How effective it is as a proxy is still up for debate. Regardless, it is still much more effective than looking at traditional statistics.
Adjusted save percentage is expressed as a 5 on 5 statistic, so penalty killing and powerplay chances are excluded. Save percentage generally drops between 4-5% while on the penalty kill, across the board. It would be unfair to penalize goaltenders that play for undisciplined teams – so it is ruled out.
There is also the myth that shot quantity has an effect on save percentage. Traditional save percentage can be fooled if a goalie is on a team that gives up a lot of shots, but they are mostly from the outside in “low-danger” zones. That was proven in a previous article, and adjusted save percentage fixes that problem by taking shot location into account.
It can also be combined with the “Goals Saved Above Average” statistic that has been discussed here on InGoal before. Stephen Burtch of Sportsnet and Nick Mercadante of Blueshirt Banter have both recently applied it to the goaltending numbers from this past season.
With adjusted save percentage, this goal…
…would have a much larger negative effect on Corey Crawford’s adjusted save percentage than his traditional save percentage because the shot came from a low-danger area.
And this save…
…would have a much larger positive impact on Kari Lehtonen’s adjusted save percentage than his traditional save percentage because the shot came from directly in front of the net.
Does this fix all of the issues with save percentage?
No, as Stephen Valiquette’s “royal road” and Chris Boyle’s “shot quality project” will tell you. While theoretically it is better to give up a shot from the blue line than in the middle of the slot – there are many other factors that go into the difficulty of a save for a goaltender.
All shots were not created equal, but we can at least account for some of the difference with adjusted save percentage. Its’ effectiveness is limited, but this is the only way to do that based on the data that is publicly collected at the moment.
Even with the advancements in the adjusted save percentage stat, goaltenders are still prone to have one fantastic year – then fall off the map the very next season. Predicting goaltenders from year-to-year is still a nightmare. When attempting to gauge their value in one season, adjusted save percentage is the way to go.
It is possible to use adjusted save percentage to judge goaltenders from different eras, but it requires an extra step. You have to equalize it compared to what the league average save percentage was that particular year, then multiply it by 100 – similar to baseball’s ERA+ statistic. Broad Street Hockey and Canucks Army both show examples of how that can be useful.
What does a good adjusted save percentage look like?
Great question! During 5 on 5 play, the league average save percentage was .923 last season – so that’s the number that goaltenders should be shooting for. (Thanks to @ for calculating those numbers for this article)
Some goalies, like Anton Khudobin in 2014-2015, see an increase in their adjusted save percentage compared to traditional. He went from a .903 unadjusted save percentage, to .910 adjusted – which shows that he was likely a victim of his team giving up many high-danger opportunities.
Other goalies, like Darcy Kuemper in 2014-2015, see their save percentage decrease after it is adjusted. His unadjusted save percentage was .907, and it went down to .898 after being adjusted. That means, if anything, he likely benefited from his team’s defensive play.
(NOTE: This section has been edited to fix an error in the data from War-on-Ice, which has since been corrected.)
Where can I find adjusted save percentage stats?
War-on-Ice collects all of the data, and it you can find it on their website here.
In conclusion, there is still a lot of work to do when it comes to statistically evaluating goaltenders – but adjusted save percentage is the best we have right now because it at least accounts for shot location. The next step will be electronically tracking players to collect more specific data about each shot that is taken during an NHL game. For now, this is it.
With the information available at our fingertips today, the only thing lazier than writing goaltenders off as “unquantifiable” would be resorting to traditional statistics like goals-against-average and wins.