Errors in judging ‘offside’ in football

Optical trickery can undermine the assistant referee's view of this ruling. In football (soccer), a player is ‘offside’ if he or she is closer to the goal than the last defender (excluding the goalkeeper) when the ball is passed to them. We investigated why assistant referees, who have the responsibility of judging offside, regularly make mistakes. We show that this is probably due to the angle of viewing by the assistant referee, who is frequently positioned beyond the last defender — a viewpoint from which errors are optically inevitable.