| Birth quartile |
Players' date of birth has been classified into four quartiles (Helsen et al., 2012): Q1 (January 1 – March 31), Q2 (April 1 – June 30), Q3 (July 1 – September 30) and Q4 (October 1 – December 31) |
| Playing Position |
The players were classified according to the different roles that occur in football such as goalkeepers, defenders, midfielders and forwards. |
| Classification |
It refers to the qualifying position of the teams in the leagues studied during the 2018/19, 2019/20 and 2020/21 seasons. The teams have been classified into three groups adapting the criteria used by Vogelbein et al. (2014): the first four places in each league (n=782), the last four places (n=777) and the middle of the table (the teams that were not in any of the previous groups, n=2538). |
