Player Selection Bias in National Football League Draftees

Beyer, KS, Fukuda, DH, Redd, MJ, Stout, JR, and Hoffman, JR. Player selection bias in National Football League draftees. J Strength Cond Res 30(11): 2965-2971, 2016 - Relative age effects (RAEs) have been studied as a potential factor associated with player selection bias in numerous sports. However, little research has examined the role of RAEs among National Football League (NFL) draftees. The purpose of the current study was to determine the existence of RAEs in NFL draftees from the last 10 NFL drafts. Draftee birth dates were collected and divided into calendar and scholastic quarters (SQ1-SQ4). To determine the presence of RAEs in specific subsets, NFL draftees were grouped according to round drafted, position, level of conference play, and age at the time of the draft. Significant χ 2 tests (p ≤ 0.05) comparing observed birth-date distributions vs. the expected birth-date distribution from the general population were followed up by calculating the standardized residual for each quarter (z > ±2.0 indicating significance). Overall, no RAEs were seen when birth-date distribution was assessed using calendar quarters (p 0.47), but more draftees were born in SQ2 (December-February) than expected (p < 0.01; z +2.2). Significantly more draftees were born in SQ2 than expected for middle-round draftees (p 0.01; z +2.4), skill positions (p 0.03; z +2.3), Power Five college draftees (p < 0.01; z +2.6), and early draftees (p < 0.01; z +3.1). However, reverse RAEs were seen among late draftees, with fewer draftees being born in SQ2 (z -3.6) and more being born in SQ4 (June-August; z +2.6) than expected. In contrast to previous research, the current study observed significant RAEs in NFL draftees from the last 10 years. This player selection bias should be considered when evaluating long-term athlete development models in American football.