Leicester's Tale: Another Perspective on the EPL 2015/16 Through Expected Goals (xG) Modelling

TL;DR

This paper develops a probabilistic expected goals (xG) model for the EPL 2015/16 season, demonstrating its effectiveness in reflecting league structure, identifying key contenders, and assessing the likelihood of rare outcomes like Leicester City's unexpected title win.

Contribution

The study introduces an inference-based xG framework that accurately captures league dynamics and evaluates rare events, providing insights into mid-season team performance and outcome probabilities.

Findings

The model accurately reflects league standings and identifies top contenders.

Leicester City's unlikely title win was statistically plausible but rare.

Mid-season xG profiles can predict second-half performance trends.

Abstract

Probabilistic modeling is an effective tool for evaluating team performance and predicting outcomes in sports. However, an important question that hasn't been fully explored is whether these models can reliably reflect actual performance while assigning meaningful probabilities to rare results that differ greatly from expectations. In this study, we create an inference-based probabilistic framework built on expected goals (xG). This framework converts shot-level event data into season-level simulations of points, rankings, and outcome probabilities. Using the English Premier League 2015/16 season as a data, we demonstrate that the framework captures the overall structure of the league table. It correctly identifies the top-four contenders and relegation candidates while explaining a significant portion of the variance in final points and ranks. In a full-season evaluation, the model assigns a low probability to extreme outcomes, particularly Leicester City's historic title win, which stands out as a statistical anomaly. We then look at the ex ante inferential and early-diagnostic role of xG by only using mid-season information. With first-half data, we simulate the rest of the season and show that teams with stronger mid-season xG profiles tend to earn more points in the second half, even after considering their current league position. In this mid-season assessment, Leicester City ranks among the top teams by xG and is given a small but noteworthy chance of winning the league. This suggests that their ultimate success was unlikely but not entirely detached from their actual performance. Our analysis indicates that expected goals models work best as probabilistic baselines for analysis and early-warning diagnostics, rather than as certain predictors of rare season outcomes.