Large Scale Analysis of Relative Age Effect on Professional Soccer Players in FIFA Designated Zones

The aim of this study was to analyse the occurrence and the magnitude of the Relative Age Effects (RAEs) in professional soccer players, and whether this effect is different depending on their position, the leagues where they play, the level of the competition, the level of the team in which they play and their nationality. The sample was composed by 12,144 professional players who participated in the professional leagues during the 2014-2015 season. The date of birth of each player was classified into four birth quartile (Q1, Q2, Q3 and Q4). Most professional soccer players are in the nearest quartiles at the beginning of the selection year, resulting in a decrease in birth as far as furthest quartiles are concerned. The RAE was found in the first (Effect Size= small) and second division (ES=medium) of competitions and in all the leagues analysed, except in the Premier League (England) and the K-League Classic (South Korea). Moreover, all playing positions (goalkeeper, defender, midfielder and forward) in soccer was affected by RAEs. In order to reduce RAE several solutions have been proposed by scientific literature that may be adopted by coaches and soccer clubs (i.e. greater number of categories, rotate court dates, or establish technical-coordinative reference values per quartile of birth).