Goalie Goals Saved Above Expected (GSAx)

Overview

This pillar analyzes a goaltender's performance beyond simple save percentage by comparing goals allowed to shot quality faced. It provides a truer measure of skill, crucial for predicting game outcomes in hockey.

What It Does

It calculates the total expected goals (xG) a goalie should have allowed based on the location, type, and context of every shot faced. This expected value is then compared to the actual number of goals conceded. A positive result indicates the goalie is performing above expectations, effectively saving goals that an average goalie would not.

Why It Matters

Traditional stats can be misleading; a goalie on a strong defensive team may face easy shots and post a high save percentage. GSAx isolates individual goalie skill from team defense, offering a more stable and predictive metric for a goalie's future performance and impact on a game.

How It Works

First, the system aggregates data for every unblocked shot a goalie faces in a given period. Each shot is assigned an expected goal value using a historical model. These values are summed to create a total expected goals against (xGA) figure. Finally, the actual goals allowed are subtracted from the xGA to determine the GSAx.

Methodology

The core formula is GSAx = Expected Goals Against (xGA) - Goals Against (GA). The xGA model is built on historical NHL shot data, considering factors like shot distance, angle from center ice, shot type, and whether the shot came off a rebound or a rush. The value is often normalized per 60 minutes of ice time (GSAx/60) for better comparison between goalies with different workloads.

Edge & Advantage

This pillar provides an edge by identifying underrated goalies on weak defensive teams or overrated goalies propped up by strong defensive systems, creating value opportunities in moneyline and totals markets.

Key Indicators

• GSAx/60

  high

  Goals Saved Above Expected, normalized per 60 minutes of play. This is the primary measure of rate performance.

• Rolling 5-game GSAx

  high

  The cumulative GSAx over the most recent five games, indicating a goalie's current form or 'hot streak'.

• High-Danger Save Percentage

  medium

  A goalie's save percentage specifically on shots with the highest probability of scoring.

Data Sources

• MoneyPuck

  Provides daily updated GSAx statistics and expected goal models for the NHL.

• Natural Stat Trick

  Offers granular shot data and situational statistics which are the building blocks for xG models.

• Evolving-Hockey

  A popular source for advanced hockey analytics, including their own proprietary GSAx model.

Example Questions This Pillar Answers

• → Will the New York Rangers win their next game against the Florida Panthers?
• → Will Connor Hellebuyck record a shutout in his next start?
• → Will the total goals scored in the Oilers vs. Canucks game be over 6.5?

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