Complex GK Union: Unveiling the unresolved aspects of shot-stopping data in Football

Marc Lamberts
10 min readJul 3, 2023

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The assessment of shot-stopping performance in football is a complex endeavor that relies on the analysis of data and statistical models. While the existing metrics have provided valuable insights into the abilities of goalkeepers, it is essential to recognise that they only scratch the surface of a much larger and intricate picture. In this article, I will embark on an exploration of the uncharted territory that lies within shot-stopping data, shedding light on the remaining gaps that have yet to be addressed and the inherent challenges that arise when attempting to bridge them.

It is important to note that the existing models and metrics have indeed made significant contributions to the understanding of shot-stopping performance. They have provided valuable tools for evaluating goalkeepers and have facilitated insightful comparisons between players. This article does not seek to discredit the work done thus far; instead, it aims to emphasise that, while these models have provided valuable information, there is still much ground to cover.

Unveiling the limitations: Pre-shot vs Post-shot xG models

In the ever-evolving landscape of football analytics, pre-shot and post-shot Expected Goals (xG) models have emerged as indispensable tools for assessing shot difficulty and evaluating goalkeepers’ performances. These models, while valuable, are not without their limitations. Pre-shot xG models overlook the goalkeeper’s influence on the outcome, while post-shot xG models struggle to isolate the goalkeeper’s specific contribution from other contextual factors. To bridge the gap between these models and unlock a more comprehensive understanding of shot-stopping data, it is necessary to refine the data collection process and develop more sophisticated algorithms that account for goalkeeper-specific variables.

Pre-shot xG models have been widely utilised to estimate the probability of a shot resulting in a goal before it is taken. These models consider various factors such as shot location, angle, distance, and assist type to determine the likelihood of a goal being scored. While pre-shot xG models provide valuable insights into shot difficulty, they often neglect the goalkeeper’s influence on altering that probability. This limitation stems from the challenge of quantifying a goalkeeper’s positioning, anticipation, and ability to close down shooting angles effectively. Refining the data collection process to capture more granular goalkeeper-specific variables, such as positioning at the time of the shot, could help bridge this gap.

On the other hand, post-shot xG models have gained prominence by evaluating the probability of a goal after the shot has been taken. These models incorporate additional variables such as shot velocity, spin, and other contextual factors to estimate the likelihood of a goal being scored based on the shot outcome. While post-shot xG models offer a more comprehensive assessment of shot quality and the goalkeeper’s performance, they face challenges in isolating the goalkeeper’s specific contribution. This difficulty arises due to the complex interactions between the goalkeeper, defenders, and other elements of the game that affect shot outcomes. Developing more sophisticated algorithms that can disentangle these interdependencies and accurately attribute shot-stopping contributions to the goalkeeper would be instrumental in bridging this gap.

To refine the data collection process and enhance the accuracy of shot-stopping evaluation, advancements in technology can play a pivotal role. High-resolution camera systems, combined with advanced tracking technologies, provide an opportunity to capture detailed positional data of goalkeepers during shot situations. By integrating this data with traditional metrics, a more nuanced understanding of shot difficulty and the goalkeeper’s influence can be achieved. Furthermore, machine learning algorithms can be leveraged to analyze vast amounts of data and identify patterns that correlate with successful shot-stopping performances. This approach would enable the development of more refined and accurate goalkeeper-specific xG models.

Beyond save percentage: The inadequacy of a singular metric

Save percentage has traditionally been the default metric used to evaluate a goalkeeper’s shot-stopping abilities in football. While it offers a straightforward measurement of saves made versus goals conceded, it falls short of providing a holistic view of a goalkeeper’s performance. The limitations of save percentage lie in its inability to account for crucial factors such as shot quality and rebound handling.

Shot quality plays a significant role in shot-stopping evaluation. Not all shots faced by a goalkeeper are equal in terms of difficulty. Some shots may be straightforward, while others are more challenging due to factors such as distance, angle, velocity, or the presence of multiple attackers. Save percentage fails to differentiate between these varying levels of shot difficulty, treating all saves and goals equally. Developing comprehensive metrics that consider shot quality would provide a more accurate assessment of a goalkeeper’s shot-stopping prowess.

The handling of rebounds is another aspect that save percentage overlooks. After making an initial save, a goalkeeper’s ability to control the rebound can be crucial in preventing follow-up shots or enabling their team to regain possession. A save that leads to a dangerous rebound can be just as detrimental as a goal conceded. Evaluating a goalkeeper’s effectiveness in controlling rebounds would bridge the gap left by save percentage, giving a more complete understanding of their shot-stopping performance.

The Complexity of Defining a “Good Save”

In the realm of shot-stopping in football, it is important to recognise that not all saves are created equal. Evaluating the quality of a save extends beyond the mere act of preventing the ball from entering the net. Several factors come into play, including positioning, reflexes, decision-making, and the ability to read the game. Closing the divide in shot-stopping data requires the development of sophisticated qualitative metrics that can capture the nuances and intricacies of what genuinely defines a “good save.”

Positioning is a fundamental aspect of shot-stopping. A goalkeeper’s ability to anticipate the trajectory of the ball and position themselves accordingly can make a substantial difference in their effectiveness. Optimal positioning allows goalkeepers to be well-placed to react to shots, increasing their chances of making successful saves. Evaluating positioning involves analysing a goalkeeper’s movement patterns, their understanding of the game flow, and their ability to position themselves to cover the most likely target areas.

Reflexes play a crucial role in shot-stopping, especially in situations where shots are taken from close range or with limited reaction time. A goalkeeper’s ability to react quickly and make instinctive saves is a key attribute in determining their shot-stopping capabilities. Assessing reflexes involves examining the speed and agility with which a goalkeeper reacts to shots, their ability to adjust their body position in mid-air, and their capacity to make athletic saves under pressure.

Decision-making is another critical factor in evaluating the quality of a save. Goalkeepers are often faced with split-second decisions, such as whether to stay on their line or come out to intercept the ball, whether to make a diving save or rely on positioning, or whether to parry the ball or hold onto it. Assessing decision-making involves analysing the goalkeeper’s ability to make sound judgments in different scenarios, considering factors such as the trajectory of the ball, the proximity of attackers, and the overall game situation.

The ability to read the game is a skill that sets apart exceptional goalkeepers. It involves anticipating the intentions of attackers, analyzing shooting angles, and understanding attacking patterns to better position themselves for saves. Reading the game encompasses both the physical and mental aspects of shot-stopping, as goalkeepers must interpret cues from their surroundings and make split-second decisions based on their analysis.

Development in shot-stopping data requires the development of sophisticated qualitative metrics that can capture these intricate elements. These metrics should go beyond traditional quantitative measures and incorporate detailed observations of positioning, reflexes, decision-making, and game reading abilities. Combining video analysis with advanced tracking technologies can assist in capturing and quantifying these qualitative aspects, enabling a more comprehensive evaluation of a goalkeeper’s shot-stopping performance.

Unmeasured Impact: Quantifying the Influence of Defending

The role of defenders in football extends beyond their primary responsibilities of marking and intercepting opponents. The defensive efforts of a team greatly influence the shots faced by goalkeepers and consequently impact the evaluation of shot-stopping performances. Factors such as the pressure applied by defenders, their positioning, and their ability to limit shooting opportunities can significantly affect shot difficulty. However, quantifying the precise effect of defending on shot-stopping remains a challenging endeavor. Bridging this gap necessitates the integration of defensive metrics with shot-stopping data to obtain a more comprehensive understanding of a goalkeeper’s performance in relation to the defensive unit.

The pressure applied by defenders plays a pivotal role in shot difficulty. A well-organised and cohesive defensive unit can disrupt the attacking flow, force opponents into making rushed or less accurate shots, and limit the space available for shooting. Conversely, a disorganised defense or gaps in the backline can lead to clear-cut scoring opportunities for the opposition. Evaluating the impact of defensive pressure on shot-stopping requires the analysis of defensive metrics such as successful tackles, interceptions, and the number of opposition passes in the defensive third. By integrating these metrics with shot-stopping data, it becomes possible to assess how effective defenders are in influencing shot difficulty and consequently the goalkeeper’s performance.

Positioning is another critical aspect influenced by defending. Defenders who maintain proper positioning can help guide attackers away from high-quality scoring positions and force shots from less advantageous angles. This can significantly impact shot difficulty and increase the likelihood of a save. Evaluating the effect of positioning on shot-stopping requires analysing defensive metrics such as the average distance between defenders and attackers, the successful execution of offside traps, and the ability to limit the space available for attackers to shoot. Integrating these metrics with shot-stopping data can provide insights into the extent to which defenders contribute to a goalkeeper’s performance.

Moreover, the ability of defenders to limit shooting opportunities can greatly influence shot difficulty. Effective defensive organization and pressing can reduce the number of shots faced by the goalkeeper, allowing them to focus on higher-quality chances. Evaluating the impact of limiting shooting opportunities requires assessing defensive metrics such as the number of blocked shots, successful clearances, and the number of opposition entries into the penalty area. By combining these metrics with shot-stopping data, a more comprehensive picture of the goalkeeper’s performance in relation to the defensive unit can be obtained.

In order to comprehensively evaluate the effect of defending on shot-stopping, it is essential to integrate defensive metrics with shot-stopping data. By considering the pressure applied by defenders, their positioning, and their ability to limit shooting opportunities, a more accurate assessment of a goalkeeper’s performance can be achieved. This integration not only provides a comprehensive understanding of shot-stopping within the context of the defensive unit but also highlights the interdependent nature of defensive and goalkeeping performances. Such insights can contribute to the development of effective defensive strategies, training regimens, and team tactics aimed at optimizing shot-stopping capabilities.

Final thoughts

In conclusion, the evaluation of shot-stopping performance in football is a multifaceted task that requires a deeper understanding of the intricacies involved. While existing models and metrics have provided valuable insights into goalkeepers’ abilities, they only scratch the surface of what can be explored. Pre-shot and post-shot xG models have made significant contributions to assessing shot difficulty, but they have their limitations in capturing the goalkeeper’s influence and isolating their specific contribution. To bridge this gap, refining the data collection process and developing sophisticated algorithms that consider goalkeeper-specific variables are necessary.

Save percentage, the traditional metric used to evaluate goalkeepers, falls short of providing a comprehensive view of their performance. Factors such as shot quality and rebound handling are not accounted for, limiting the metric’s ability to accurately assess a goalkeeper’s shot-stopping prowess. Developing comprehensive metrics that consider these factors will lead to a more accurate evaluation of a goalkeeper’s performance.

Furthermore, evaluating the quality of a save goes beyond simply preventing a goal. Factors such as positioning, reflexes, decision-making, and game reading abilities all play significant roles in determining the quality of a save. Developing sophisticated qualitative metrics that can capture these nuances is crucial to obtaining a more complete understanding of shot-stopping performance.

Defensive efforts have a substantial impact on the shots faced by goalkeepers. Factors such as defensive pressure, positioning, and the ability to limit shooting opportunities greatly influence shot difficulty. However, quantifying the precise effect of defending on shot-stopping remains a challenge. Integrating defensive metrics with shot-stopping data is necessary to gain a more comprehensive understanding of a goalkeeper’s performance within the context of the defensive unit.

In the pursuit of a more comprehensive evaluation of shot-stopping performance, advancements in technology, such as high-resolution camera systems and advanced tracking technologies, can provide detailed positional data and enhance the accuracy of data collection. Machine learning algorithms can also be leveraged to analyse vast amounts of data and identify patterns that correlate with successful shot-stopping performances.

Overall, the field of shot-stopping data analysis in football is still evolving, and there are many unexplored areas that require further investigation. By addressing the limitations of existing models, incorporating comprehensive metrics, and integrating defensive metrics, we can bridge the gaps in shot-stopping data and gain a more nuanced understanding of a goalkeeper’s performance. Such advancements will not only benefit individual goalkeepers but also contribute to the development of effective defensive strategies and team tactics in the pursuit of success on the football pitch.

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Marc Lamberts
Marc Lamberts

Written by Marc Lamberts

Academic | CAF A | Recruitment + data analysis consultant in football | Set pieces

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