GSAA: An Essential Statistic For Evaluating Goaltenders
Advanced statistical knowledge in sports has increased rapidly in the last two decades. Hockey has been a step behind most other sports, but is starting to catch up with the rise of possession statistics such as the wildly popular “Corsi” percentage. The sheer volume of information that is collected now can be enough to make your head spin, but a lot of it is extremely valuable when it comes to evaluating individual performance.
When it comes to evaluating a goaltender, three statistics in particular have been looked at throughout most of history. Those three stats are wins, goals against average and save percentage. While all three categories still have some merit, they have become somewhat outdated. Wins and goals against average are better looked at as a team statistic and is actually a very poor way to evaluate an individual goaltender’s performance. Even save percentage isn’t perfect. It can be skewed depending on the quality of team that the goaltender plays for.
In baseball, batting average, home runs and runs batted in were the “big three” categories for years. They have been effectively replaced by percentage stats, and statistics that compare a player to a “replacement player.” Essentially, if that player was replaced by a minor leaguer, would that minor league player perform better, or worse? It’s called “WAR” which stands for Wins Above Replacement.
There is a relatively new statistic for hockey that has been made available by the folks at Hockey-Reference. It’s similar to baseball’s WAR, and it is called “GSAA” – Goals Saved Above Average. You take the league’s average save percentage and apply it to the amount of shots a particular goalie has faced. You get a number of goals that the average goalie in that league would have surrendered if they faced the same number of shots as the goaltender in question. That number gets compared to the number of goals surrendered by that goaltender, and a plus/minus is created. If a goalie is in the positive, that is how many goals they have saved compared to a league-average goalie. If they are in the negative, then it is safe to assume that they are performing worse than how a league-average goaltender would perform in the same situation.
Here are the top ten goaltenders in GSAA in the 2013-14 NHL season, as provided by Hockey-Reference:
For the full list of GSAA leaders, click here.
Every stat has pros and cons, which is why you must look at the entire picture. GSAA does a lot of things better than other stats, but is still not perfect. Here is a breakdown of what it does well, ways that it is biased, and how it can be improved:
What GSAA Does Well
- It is a very good stat at equalizing goalies across the league, regardless of the team that they play for. For example, a goalie that has a .925 save percentage and has faced a lot more shots than average is actually playing better than a goalie that has a .930 save percentage and has faced less shots. The first goalie has faced more scoring opportunities, and has saved more goals from going in based on their quality of play.
- It tells you how much a team relies on their goaltending to win games. If a team gives up a lot of shots, but their goalie is continually bailing them out, their goalie will have a very high GSAA number. Those teams are more likely to struggle if their goalie goes into a slump or gets injured. Teams that succeed despite having a goaltender that is in the middle of the pack (or worse) in GSAA are actually more stable, because if their goalie slumps or gets injured, they still have a very good chance to win using a replacement netminder.
- It gives you a physical number of goals saved, rather than a percentage. It’s a stat that can be very shocking. For example, Ben Bishop has saved almost 24 goals from being scored on the Tampa Bay Lightning in 44 games. A number that large will draw a lot of attention. It is a great stat to prove Bishop’s worth to the Lightning. 24 goals saved is a VERY significant number.
What GSAA Does Not Do Well
- Goalies that play more games will accumulate more goals saved. If the stat was expressed as GSAA/per 60 minutes, it would be even more accurate. You would have to turn it into a percentage, though. Only goalies that have played a certain number of games would be able to qualify.
- It does not take penalty killing into account. On average this season in the NHL, goalies have seen a 4.4% drop in save percentage while on the penalty kill compared to even strength. This is a major problem for some goalies that play for a team that is constantly killing penalties. Undisciplined teams will have goalies with a lower save percentage and a lower GSAA as a result.
- It does not take fatigue into account. Saves that are made after the 30 shot mark should be worth more, because goalies that face more shots than the league average will be more tired, and will make less saves due to the poor quality of the team in front of them giving up more scoring attempts.
- It also does not take shot quality into account. This can be related to the number of penalties a certain team commits, or even a team’s quality of defence, but that would be very difficult for any statistic to quantify.
GSAA is definitely not a perfect statistic, but it is one of the best ones available at the moment when it comes to analyzing goaltenders. It has flaws, but it is more accurate than save percentage and should become more widely used.