The first objective of the present study was to examine the accuracy of offside judgements of assistant referees during the 2006 World Cup and to compare overall results with performances of assistant referees during the 2002 World Cup. Second, we also examined underlying mechanisms leading to incorrect decisions. According to the definition used for the 2002 World Cup, the results revealed a decrease in the number of incorrect flag signals during the 2006 (24/240 or 10.0%) compared with the 2002 World Cup (58/222 or 26.1%) (P < 0.001). For accuracy of all potential offside situations, according to a new definition (with the attacker 2 m in front or behind the second-last defender), the error percentage was 7.6% (17 flag errors and 9 non-flag errors out of 342 situations). Overall, quality of offside decisions was not influenced by the position of the assistant referee relative to the offside line. In addition, flag errors can best be explained by the perceptual illusion induced by the flash-lag effect.
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