31 Jul Evaluation of Goalkeeper Quality in LaLiga
Introduction
Evaluating the quality of goalkeepers is a crucial aspect of analyzing defensive performance in football. Historically, metrics used to evaluate goalkeepers, such as the number of saves and save percentage, have been insufficient in capturing the complexity of their performance. This study proposes a combined metric that incorporates multiple factors for a more comprehensive and accurate evaluation of goalkeeper quality in LaLiga from the 2008/2009 season to the 2023/2024 season.
Methodology
The proposed metric combines several key elements:
- Inverse Normalized Goals Prevented per SOT: This component adjusts the «Goals Prevented per SOT» (Shots On Target) metric using an inverse normalization factor based on the number of shots on target per opponent possession. This ensures that teams facing fewer shots due to solid defense are not disadvantaged in the evaluation.
- Goals Prevented per SOT measures the difference between the expected goals (PSxG) faced and the actual goals conceded, normalized by the number of shots on target (SOT) faced.
- SOT per Opponent Possession calculates the frequency of shots on target faced per possession of the opposing team, providing a measure of defensive efficiency in reducing shooting opportunities.
- The Inverse Normalized Goals Prevented per SOT then multiplies the goals prevented per SOT by the inverse of the SOT per opponent possession, creating a more balanced measure that accounts for both the quality and quantity of shots faced.
- Clean Sheets: This component includes the number of matches in which the team did not concede any goals, normalized by the total number of matches played. It reflects the ability of the goalkeeper and defense to consistently prevent goals.
The final metric, called Combined Goalkeeper Quality, is scaled from 0 to 100 for ease of interpretation, where 100 represents the highest possible quality and 0 the lowest.
Results
Evolution of Goalkeeper Quality in LaLiga The graph below shows the evolution of the goalkeeper quality metric from the 2008/2009 season to the 2023/2024 season, highlighting a significant improvement in goalkeeper quality over time.
Top 10 Goalkeeper’s Teams by Number of Times in Top 5 (2008/2009 – 2023/2024) This graph shows the teams whose defenses and goalkeepers have consistently excelled, appearing in the Top 5 of our metric multiple times.
Top 5 Goalkeeper’s Teams in 2023/2024 Season In the 2023/2024 season, the following teams stood out in our goalkeeper quality metric, reflecting superior performance.
Discussion
Using a combined metric allows for a more comprehensive evaluation of goalkeeper quality, considering both the ability to make saves and the overall team’s effectiveness in preventing high-quality shots. The inclusion of factors such as clean sheets and the adjustment for opponent possessions adds a critical dimension, enabling better differentiation between goalkeepers and teams facing varying volumes and qualities of shots.
Conclusions
This study provides a new tool for evaluating the quality of goalkeepers in LaLiga. The combined and scaled metric allows for a clear and fair comparison, reflecting both the individual goalkeeper’s performance and the team’s defensive capabilities. The results indicate an improvement in goalkeeper quality over the last 15 seasons, highlighting teams that consistently excel in this area.