Goalie Stats 13 - Goals Saved Above Expected (GSAx)
- tmlblueandwhite
- Dec 24, 2022
- 2 min read
Updated: Jan 28, 2023
GSAx - ANALYZING GOALIE CHANGES
(April 8, 2021)
The reasoning for using GSAx, is that you want to factor out team defense as much as possible.
So, the formula for GSAx is expected goals against minus goals against.
The primary reason why SV% and GAA are less effective than GSAx is that they fail to isolate the goaltender, much like wins.
The beauty of GSAx is that it is an all-encompassing metric. Rather than looking at some combination of the mainstream stats to determine the league’s top goalies, one can simply pluck the name at the top of the GSAx leaderboard because it truly does isolate goaltender performance in the most effective way we have at our disposal.
In a single game, anything above 1.00 GSAx is a great performance. Any game around 3.00 GSAx is an incredible performance and likely a stolen game.
Over the course of a full season, a high-quality starting goaltender will get around 10.00 GSAx. A Vezina-worthy performance, in most seasons, is likely around 20.00 GSAx and above.
adjGSAxGA/60
(July 30, 2016)
Where adjGSAxGA/60 splits from adjGSAA/60 is in how, instead of weighting by danger zones and using only shots on goal, we’re going to account for missed shots as well and adjust for Emmanuel Perry’s xG model.
how well does a goaltender both save shots on goal and influence shooters to miss wide if he faces league-average shot quality?
GSAx vs GSAA
There are generally two ways to judge goalies. You either go with goals saved above average (GSAA) or you go with goals saved above expected (GSAx) which is based on xGA. GSAA relies solely on the number of shots against and the number of goals allowed. GSAx is the difference between expected goals against and actual goals against, so it depends on the number of shots and the quality of the shots. The problem is that xG-models aren’t perfect when it comes to evaluating goaltending.
The truth probably lies somewhere in between GSAA and GSAx. I honestly don’t think there’s any good way to measure goaltending as of right now. It’s almost impossible to distinguish between good goaltending and good team defense. The new upcoming tracking data may lead to a better goalie stat. Until then, this is what we got.
some goalies are likely overvalued if you judge them by LS-GAA or GSAx, while others are undervalued. I don’t think GSAA is any better though, so perhaps the smartest way to evaluate goalies is by combining the two.
Goaltending is more or less unpredictable with the current metrics, but at the same time goaltending is hugely important. So if you can somehow predict future goaltender performance, then you can make a great projection model.
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