Off-side calls from the 2010/11 first German division soccer games were analysed. It was predicted that the accuracy of off-side decisions would be less when the angle between the passer and the last defender was higher. In other words, when the passer and the last defender are further away from each other (hence increasing the visual angle), accuracy in offside decisions will be worse.
Results supported the hypothesis. When the visual angle was < 40 degrees, only 11% of offside calls were incorrect. Conversely, when the visual angle was >40 degrees, 30% of offside calls were incorrect.
In a second study, 10 professional referees volunteered to have their attention breadth measured (i.e., the maximum visual angle where they can attend to two objects). This meant that the analysis could take into consideration individual differences in attention breadth. Nonetheless, results revealed the same trend. More incorrect offside calls were made when visual angle increased.
What does this mean for assistant referees in soccer? Clearly it is more accurate to make offside calls when standing further away from the pitch, as this reduces the visual angle between the passer and last defender. No wonder spectators appear to see the offside better! However, obviously standing further away from the pitch is impractical in professional soccer. Soccer authorities should seek to develop strategies that aim to limit the visual angle problem from resulting in erroneous offside calls.