Flames Goalies: Goals Saved Above Expected (GSAx)

In the modern National Hockey League, evaluating goaltending extends far beyond traditional statistics like wins and save percentage. Advanced metrics now provide a deeper, more nuanced understanding of a goaltender's true performance by accounting for the quality of shots faced. This glossary breaks down the key terms and concepts surrounding Goals Saved Above Expected (GSAx), the essential metric for analyzing the performance of Calgary Flames netminders like Jacob Markström and his counterparts across the league.

Goals Saved Above Expected (GSAx)

A cumulative advanced statistic that measures a goaltender's performance against the quality of shots faced. It calculates the difference between the number of goals a goalie has actually allowed and the number of goals an average NHL goaltender would be expected to allow given the location and type of all shots faced. A positive GSAx indicates a goalie is performing better than average, while a negative value suggests below-average performance.

Expected Goals Against (xGA)

The total number of goals an average NHL goaltender would be expected to surrender based on the cumulative quality of all shots faced. This model considers variables such as shot distance, angle, shot type (e.g., wrist shot, slap shot), and whether the shot followed a rebound. It provides the baseline against which a goalie's actual goals against are compared.

Shot Quality

A measure of the likelihood that a shot from a specific location and situation will result in a goal. Shots from the slot or off a cross-crease pass have high quality (a high probability of becoming a goal), while unscreened wrist shots from the perimeter have low quality. GSAx models are built on extensive historical data to assign a probability to every shot.

High-Danger Scoring Chance (HDSC)

A shot attempt taken from the area of the ice where goals are most frequently scored, typically the slot and the inner portion of the face-off circles. Saves made on these chances are weighted more heavily in xGA models and are critical for a strong GSAx. Consistently stopping HDSCs is a hallmark of elite goaltending.

Low-Danger Shot

A shot attempt originating from outside the high-danger areas, such as the points or sharp angles below the goal line. While these shots have a low probability of scoring, volume can impact a goalie's traditional save percentage. A goalie's ability to avoid "soft goals" on these shots is foundational.

Save Percentage (SV%)

A traditional statistic calculated by dividing saves by total shots on goal. While useful, it does not account for shot quality. A goalie facing many low-danger shots can have a deceptively high SV%, while a goalie consistently battling high-quality chances might have a lower, less indicative percentage.

Adjusted Save Percentage (SV% adj.)

An enhanced version of save percentage that weights saves based on the shot quality. It provides a more accurate reflection of a goaltender's skill by adjusting for the difficulty of their workload, making it a better companion stat to GSAx for cross-comparison.

Fenwick Save Percentage (FSV%)

A variant of save percentage that considers all unblocked shot attempts (shots on goal plus missed shots). The theory is that it provides a larger sample size and may better capture a goalie's true performance, though it is less commonly cited than SV% in public discourse.

Goals Against Average (GAA)

The average number of goals a goaltender allows per sixty minutes of play. Like SV%, it is a team-dependent stat heavily influenced by defensive play and systems in front of the goalie, making it an imperfect measure of individual goaltender performance.

Quality Start

A game in which a goaltender achieves a save percentage above the league average (approximately .912) or allows two or fewer goals while posting a save percentage above .885. For the Flames, a Quality Start from Markström is often a prerequisite for securing two points.

Really Bad Start

A game in which a goaltender posts a save percentage below .850. These starts are significant negative outliers that can drastically impact a goalie's cumulative GSAx over a short sample size, such as a segment of the 2023-24 NHL season.

Goals Saved Above Average (GSAA)

A precursor to GSAx, this metric compares a goalie's performance to the league-average save percentage on the same number of shots. It is simpler than GSAx as it does not account for shot quality, using a flat league-average baseline instead.

Public Models (Evolving-Hockey, MoneyPuck, Natural Stat Trick)

Different analytics websites that publish their own proprietary xGA and GSAx models. While their core principles are similar, slight variations in data sources and model weighting can lead to different numerical results for the same goaltender.

Rink Bias

The potential for systematic error in shot location data collected in different arenas. This is a known challenge in building league-wide models, as the precision of location tracking can vary from venue to venue, including the Scotiabank Saddledome.

Contextual Factors

Elements not captured in standard GSAx models that can affect shot quality. These include pre-shot movement, screen traffic, defensive breakdowns, and game state (e.g., power play, penalty kill). Coaches like Ryan Huska use this context to evaluate performance beyond the raw number.

Cumulative vs. Rate Statistic

GSAx is primarily a cumulative stat, meaning it adds up over the season. A GSAx "per 60 minutes" rate can also be calculated to compare goalies with different amounts of playing time, offering a view of performance density.

Roster Construction

The process by which management, led by GM Craig Conroy, builds a team. GSAx is a critical data point in this process, helping to evaluate if current goaltending is a strength, to assess trade targets, or to project the readiness of prospects.

Performance Variance

The natural fluctuation in a goaltender's results over time. Even elite starters will have stretches with negative GSAx. The key for the Flames is managing this variance through workload, practice, and video review to maximize consistency.

Defensive Structure

The systematic approach a team uses to limit high-quality shots against. A sound structure, as implemented by Huska, can suppress the volume of high-danger chances, making a goalie's job easier and positively influencing his GSAx.

Rebound Control

A goaltender's ability to direct shots to low-danger areas of the ice or to freeze the puck. Poor rebound control leads to second-chance opportunities, which are often high-danger shots and can quickly degrade a goalie's GSAx.

Post-Shot Expected Goals (PSxG)

An evolution of the xG model that factors in the shot's trajectory after it leaves the stick, not just its initial location. This can account for screens, deflections, and shot velocity, providing an even more precise measure of shot quality and expected goals.

Goaltender Workload

The total number of shots and high-danger chances a goalie faces over a period. A heavy workload, especially in a condensed schedule, can lead to fatigue, potentially impacting performance metrics like GSAx over the grueling Western Conference schedule.

Market Efficiency

The concept that as GSAx and similar metrics become mainstream, their predictive value in assessing player value and contract worth may change. A strong GSAx can significantly influence a goalie's market value at the trade deadline or in free agency.

The Eye Test

The qualitative evaluation of a goaltender's technique, positioning, and composure. While GSAx provides quantitative evidence, the eye test from scouts and coaches remains vital for a holistic assessment, especially for developing players.

Understanding Goals Saved Above Expected is fundamental for any serious analysis of the Calgary Flames' fortunes. It moves the conversation beyond simplistic stats, offering a data-driven lens to evaluate whether Jacob Markström is stealing games, if the defensive system is functioning, and how the goaltending position stacks up in the competitive Pacific Division. For Flames fans in the C of Red, it provides a deeper appreciation for the critical saves that define close games in the Battle of Alberta and the long journey of an NHL campaign.

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