Did Lille, Atletico or Leicester win their respective leagues riding on their luck? Possibly not.

What is it about?

It is common wisdom that luck plays a role in sports, along with skill. But what constitutes luck? There is no empirical study to assess the role of luck in the determination of rank positions in football (soccer) leagues. In this paper, we define X-factor as unforeseen and unaccounted factors and quantify it as the difference between actual and predicted values of performance or outcome variables. For league football, we have perceived the difference between actual and expected goal difference as the X-factor effect in performance, and the difference between actual and expected points as the X-factor effect in outcome. Further, we have ideated that a plausible role of luck cannot be ruled out if the X-factor effect on outcome is significant while that on performance is not. Conducting analyses of variance on observations from seven seasons (2014–15 to 2020–21) in the top tier leagues of England, Spain, Germany, Italy, and France, we detected the presence of a significant and systematic X-factor effect. We have studied the role of luck using Tukey’s HSD test. In general, luck does not play any significant role in determining the rank positions in league football.

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Why is it important?

This study is the first of its kind where X-factor effect has been estimated. None of the earlier studies evaluated the role of luck in football leagues, using the xG model. In view of this uniqueness, this paper makes an important contribution in sports analytics and can potentially open a new area of research.

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