The objective of this study was to analyse the correctness of the offside judgements of the assistant referees during the final round of the FIFA 2002 World Cup. We also contrasted two hypotheses to explain the errors in judging offside. The optical error hypothesis is based on an incorrect viewing angle, while the flash-lag hypothesis refers to perceptual errors associated with the flash-lag effect (i.e. a moving object is perceived as spatially leading its real position at a discrete instant signalled by a briefly flashed stimulus). Across all 64 matches, 337 offsides were analysed using digital video technology. The error percentage was 26.2%. During the first 15 min match period, there were significantly more errors (38.5%) than during any other 15 min interval. As predicted by the flash-lag effect, we observed many more flag errors (86.6%) than non-flag errors (13.4%). Unlike the predictions of the optical error hypothesis, there was no significant difference between the correct and incorrect decisions in terms of the positioning of the assistant referees relative to the offside line (0.81 and 0.77 m ahead, respectively). To reduce the typical errors in judging offside, alternative ways need to be considered to teach assistant referees to better deal with flash-lag effects.

The offside decision-making process of international and national assistant referees (ARs) was evaluated using video simulations. A Tobii T120 Eye Tracker was used to record the eye movements. Two hypotheses for explaining incorrect decisions were investigated, namely, the flash-lag effect and the shift of gaze. Performance differences between skill levels were also examined. First, results showed a bias toward flag errors for national ARs as expected by the flash-lag effect. Second, ARs fixated the offside line before, during, and after the precise moment the pass was given, implying there was no shift of gaze from the passer to the receiving attacker. Third, no differences were found in scan patterns between international and national ARs. In conclusion, international ARs seem to have found a strategy to better deal with the perceptual illusion resulting from the flash-lag effect. Based on their experience, they have learned to correct for this illusion, and, consequently, show fewer flag errors.

This study investigated the offside decision-making process in association football. The first aim was to capture the specific offside decision-making skills in complex dynamic events. Second, we analyzed the type of errors to investigate the factors leading to incorrect decisions. Fédération Internationale de Football Association (FIFA; n ϭ 29) and Belgian elite (n ϭ 28) assistant referees (ARs) assessed 64 computer-based offside situations. First, an expertise effect was found. The FIFA ARs assessed the trials more accurately than the Belgian ARs (76.4% vs. 67.5%). Second, regarding the type of error, all ARs clearly tended to raise their flag in doubtful situations. This observation could be explained by a perceptual bias associated with the flash-lag effect. Specifically, attackers were perceived ahead of their actual positions, and this tendency was stronger for the Belgian than for the FIFA ARs (11.0 vs. 8.4 pixels), in particular when the difficulty of the trials increased. Further experimentation is needed to examine whether video-and computer-based decision-making training is effective in improving the decision-making skills of ARs during the game.

In association football, two similar but arguably different refereeing roles are required, those of the referee and assistant referee. Role specificity was investigated with a foul play assessment task and an offside decision-making task. Deliberate practice was investigated to account for role-specific differences. First, role specificity was clearly observed. Second, years of officiating, hours of practice per week, and number of matches officiated were each positively correlated with skill. The results support role specificity in association football refereeing. Further research should help to create role-specific perception and decision-making training programmes both for referees and assistant referees.