Injury time bias, also known as +/- bias, is a type of favoritism shown by soccer referees. Injury time is the extra time added to the first half after 45 minutes and to the second half after 90 minutes, mostly due to player injuries or other delays.
A study by Sutter and Kocher (2004) revealed that referees tend to add more injury time during matches when the home team is losing by one goal compared to when they are winning by one goal after 90 minutes. This bias is thought to be influenced by unconscious social pressure, like the influence of the crowd.
Researchers have looked into this bias in various top soccer leagues in different countries. For example, in Spain, there was an average of 112 seconds of extra time, while in Italy it was 30 seconds, in the United States 8 seconds, in England 31 seconds, and in Brazil 11 seconds. In Germany, the mean difference was 18 seconds. However, interestingly, Riedl et al. (2015) showed that this bias does not actually affect the home advantage in football.
Riedl, D., Strausß, B., Heuer, A., & Rubner, O. (2015). Finale furioso: Referee biased injury times and their effects on home advantage in football. Journal of Sports Sciences, 33, 327 336.
Sutter, M., & Kocher, M. G. (2004). Favoritism of agents The case of referees’ home bias. Journal of Economic Psychology, 25, 461 469.
