Goaltender GSAx (Goals Saved Above Expected)

Overview

This pillar analyzes a goaltender's performance beyond simple save percentage by measuring Goals Saved Above Expected (GSAx). It accounts for shot quality, providing a true measure of a goalie's ability to win games.

What It Does

Goaltender GSAx evaluates every shot a goalie faces, using an Expected Goals (xG) model to determine the probability of each shot resulting in a goal. It considers factors like shot location, type, and pre-shot movement. The pillar then compares the expected goals against to the actual goals allowed, revealing if a goalie is performing above or below expectations.

Why It Matters

Traditional stats like save percentage can be misleading, as they are heavily influenced by the team's defensive play. GSAx isolates the goalie's individual contribution, identifying elite goaltenders who can steal games and flagging those who are overperforming due to a strong defense.

How It Works

First, shot data for a game or season is collected, including location and type. An Expected Goals (xG) model assigns a goal probability to each shot. These probabilities are summed to create a total Expected Goals Against (xGA). Finally, GSAx is calculated by subtracting the actual Goals Against (GA) from the xGA.

Methodology

The core formula is GSAx = Expected Goals Against (xGA) - Goals Against (GA). The xGA model incorporates shot location, shot type, pre-shot puck movement, and game state (e.g., 5-on-5, power play). Analysis typically uses a rolling 25-game window at 5-on-5 to provide a stable, recent performance sample.

Edge & Advantage

This pillar provides an edge by identifying overvalued or undervalued goalies whose traditional stats are skewed by their team's defensive performance.

Key Indicators

• GSAx/60

  high

  Goals Saved Above Expected per 60 minutes of play, normalizing performance for goalies with different amounts of ice time.

• High-Danger Save Percentage

  high

  A goalie's save percentage specifically on shots from high-probability scoring areas, indicating performance under pressure.

• Rebound Control Rating

  medium

  A metric assessing a goalie's ability to prevent dangerous rebounds, which limits second-chance scoring opportunities.

Data Sources

• MoneyPuck

  Provides daily updated GSAx and Expected Goals models for all NHL games and players.

• Natural Stat Trick

  Offers detailed 5-on-5 and all-situations hockey analytics, including shot quality and goaltending metrics.

• Evolving-Hockey

  A subscription service offering advanced, customizable hockey models including their own GSAx calculations.

Example Questions This Pillar Answers

• → Will the New York Rangers win against the Boston Bruins tonight?
• → Will the total goals in the Oilers vs. Flames game be over 6.5?
• → Will Andrei Vasilevskiy have a save percentage over .915 in his next game?

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