title: The relative age effect in a professional football club setting.
abstract: The relative age effect is an uneven distribution of birth date favouring subjects born in the initial months of a selection year. This study compared the birth-date distributions between several subgroups of Basque football players to identify whether the relative age effect is influenced by age and/or skill level. The study comprised 13,519 players including 114 senior professionals from the Spanish league's AC Bilbao over 21 seasons; over the season 2005-2006, it comprised elite youth (n=189) from the same club's academy; regional youth (n=4382) U11-U14 locally federated players; school youth (n=8834) U10-U11 locally registered school district players. Differences between the observed and expected birth-date distributions were tested based on data from the general Basque male population. Significant chi-square values were followed up by calculating odds ratios and 95% confidence intervals (CI) for the quartile and half-year distributions to examine subgroup differences in the relative age effect. Birth-date distributions of all groups of players showed a significant bias towards early birth in the selection year compared with the reference population (senior, chi-2(3) = 24.4, P < 0.001; elite youth, chi-2(3) = 59.1, P < 0.001; regional youth, chi-2(3) = 41.4, P < 0.001; school youth, chi-2(3) = 40.9, P < 0.001). Between-group comparison revealed that the relative age effect incidence progressively increased with a higher level of involvement in youth football. This bias represents a significant loss of potential youth football talent.
authors.affiliation: USP Araba Sport Clinic, Vitoria-Gasteiz, Basque Country, Spain.
authors.forename: Iñigo
authors.initials: I
authors.lastname: Mujika
authors.forename: Roel
authors.initials: R
authors.lastname: Vaeyens
authors.forename: Stijn P J
authors.initials: SP
authors.lastname: Matthys
authors.forename: Juanma
authors.initials: J
authors.lastname: Santisteban
authors.forename: Juan
authors.initials: J
authors.lastname: Goiriena
authors.forename: Renaat
authors.initials: R
authors.lastname: Philippaerts
mesh_terms: D000293:Adolescent; D000367:Age Factors; D054874:Athletic Performance; D002648:Child; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D016017:Odds Ratio; D012916:Soccer; D055815:Young Adult