Why it’s better ‘out’ than ‘in’ when querying line calls

Author: By Steve Connor, Science Editor

The human eye and brain have a tendency to see a bouncing tennis ball landing outside the court rather than inside the line, which is why referees are more likely to make mistakes when deciding whether a ball is out, scientists said. The findings can be exploited tactically by tennis players, who can only challenge a decision a limited number of times. Researchers say players should dispute controversial “out” calls and ignore “in” calls, even if the player thinks the “in” call is wrong.

The findings have emerged from research into how the human visual system monitors a moving object. The eye tends to see a moving object bounce further in the direction of the object’s movement. In tennis, there are two kinds of errors, primarily made when a ball is called in or out of play. The ball could bounce in the court but be called out, or bounce out and be called in, the scientists said. If tennis referees were bias-free they would be equally likely to make each of these two errors. However, the study of line-call errors at Wimbledon found a significant bias for wrong “out” calls.

From more than 4,000 tennis points chosen at random from the past two Wimbledon tournaments, the scientists found 83 incorrect line calls. Of these, 70 fell in to the predicted pattern, indicating that the referees suffered from the human perceptual bias.

“The visual system faces a big challenge when trying to code the locations of objects so we can perceive them,” said David Whitney of the University of California, Davis, who led the study, published today in the journal Current Biology. Audiences and tennis players themselves are subject to the inherent bias of the human visual system, which is why players should concentrate on challenging disputed “out” calls.

“Because players are allowed to continue challenging calls as long as the challenges are correct, players should predominantly challenge those calls that are consistent with the perceptual error revealed here,” the scientists said. “Players who make better use of their challenges benefit more.”

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