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Judging offside in football. Journal Club, 2006. Errors in judging ‘offside’ in football. Oi Lino – are you blind?. Law 11: Offside. A player is in an offside position if he is nearer to his opponents goal line than both the ball and the second last opponent

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judging offside in football

Judging offside in football

Journal Club, 2006

law 11 offside
Law 11: Offside
  • A player is in an offside position if he is nearer to his opponents goal line than both the ball and the second last opponent
  • A player in an offside position is only penalised if, at the moment the ball touches or is played by one of his team, he is, in the opinion of the referee involved in active play by:
    • interfering with play
    • interfering with an opponent


oudejans et al 2000 nature
Oudejans et al (2000) - Nature
  • Claim angle of viewing by assistant referee leads to inevitable optical errors.
  • 3 assistant referees.
  • 200 offside judgements - 40 errors.
  • ARs cannot see passer and receiver simultaneously.
  • ARs 1.2m ahead of offside line on average
cause of error
Cause of error
  • One hypothesis:
    • AR shifts gaze from passer to receiver
  • Alternate hypothesis:
    • Misalignment of AR and last defender (1.18m)
    • Flag error (FE) - receiver wrongly perceived as ahead of last defender
    • No flag error (NFE) - receiver wrongly perceived as behind last defender


FE/NFE: 171/31



FE/NFE: 21/43

FE/NFE: 171/31


FE/NFE: 34/16


FE/NFE: 19/84

FE/NFE: 34/16

  • FE bias when attackers go left.
  • NFE bias when attackers go right.
  • Errors caused by relative optical projections of players on AR’s retina.
  • AR limited by their perceptual systems (or positioning systems!).
  • Off-line analysis of video images from adequate observation point.
baldo et al 2002 perception
Baldo et al (2002) - Perception
  • Reanalysis of Oudejans et al’s data.
  • Proposed that flag errors due to flash-lag effect rather than geometrical/optical effect.
  • Flash-lag effect:
    • A moving object is perceived as spatially ahead of its real position at an instant defined by a time marker (e.g. a flash)
flash lag model for football
Flash-lag model for football
  • Moving object: the receiver running towards goal.
  • Time marker: the moment that the passer kicks the ball.
  • This effect adds to the geometric/optical effect, leading to an overall bias towards FE than NFE
  • FE/NFE ratio was 324/240
  • Asymmetry in left & right trajectories (FE bias for left in geometric/optical effect).
  • Flash-lag effect predicts increase in FE for both left & right trajectories:
    • (NFE/FE)LT < (FE/NFE)RT.
    • (65/266)LT < (58/175)RT.
  • Flash-lag contributes to an existing geometric/optical effect.
  • Other factors may contribute (over-zealousness, FIFA recommendations etc.).
  • Flash-lag errors could also occur when the AR is in alignment with the offside line.
  • Bridge between lab and field needs to be made.
  • Eagleman: flash-lag occurs when flash & moving object appear in same location.
  • Can FL occur when flash is in peripheral?
  • Variance in relationship/distance between attacker & defender (e.g., far/near)?
  • Gaze shift hypothesis rejected after examination of 1 AR!
  • No gaze shift - how do ARs fulfil other responsibilities (e.g., throw-in)?