Table 2. Definition of the variables. Variable Definition Birth quartile The date of birth of the players has been classified in 4 quartiles (Helsen et al., 2012, Romann and Fuchslocher, 2013): Q1 (1st of January – 31st of March), Q2 (1st of April – 30th of June), Q3 (1st of July – 30th of September) and Q4 (1st of October – 31st of December) Playing Position The players were classified according to the different roles that are given in football: goalkeepers, defenders, midfielders and strikers Classification It refers to the qualifying position of the teams of the leagues studied during the 2016-2017 season. The teams have been classified into 3 groups adapting the criteria used by Vogelbein et al. (2014): high (the first 4 teams of each league, n = 40), media (the teams that are neither between the first 4, nor last 4, n = 98) and low (the last 4 teams of each league, n = 40).