The effects of draw restrictions on knockout tournaments

TL;DR

This paper investigates how draw restrictions, like avoiding same-country matchups, affect the outcomes of knockout tournaments through theoretical and simulation models, revealing increased chances for favorites and implications for real-world competitions.

Contribution

It provides a combined theoretical and simulation analysis of draw constraints' impact on tournament outcomes, especially regarding favorite team advantages.

Findings

Association constraints increase same-nation matchups in subsequent rounds.

Favorites from concentrated associations have higher winning probabilities.

Constraints' effects are less than linear when applied over multiple rounds.

Abstract

The paper analyses how draw constraints influence the outcome of a knockout tournament. The research question is inspired by European club football competitions, where the organiser generally imposes an association constraint in the first round of the knockout phase: teams from the same country cannot be drawn against each other. Its effects are explored in both theoretical and simulation models. An association constraint in the first round(s) is found to increase the likelihood of same nation matchups to approximately the same extent in each subsequent round. If the favourite teams are concentrated in some associations, they will have a higher probability to win the tournament under this policy but the increase is less than linear if it is used in more rounds. Our results might explain the recent introduction of the association constraint for both the knockout round play-offs with 16 teams and the Round of 16 in the UEFA Europa League and UEFA Europa Conference League.