The effects of draw constraints in knockout tournaments

Abstract

The paper aims to uncover some implications of draw restrictions used in knockout tournaments. The motivation is provided by the UEFA Europa League, the second-tier competition of European club football, which has been dominated recently by two countries: among the 12 winners (24 finalists) between the 2009/10 and 2020/21 seasons, three (seven) and eight (nine) teams have come from England and Spain, respectively. According to the rules of this tournament, clubs from the same country could not have been drawn against each other in the group stage and the Round of 32. However, two clashes between English and nine clashes between Spanish teams have taken place in the subsequent rounds during this period.