It’s normally football that is known for dramatic challenges against the referee’s decision, but new research out of the University of California, Davis suggests that things may be about to change. A team at The Department of Psychology and the Center for Mind and Brain found that professional tennis referees are more likely to call a ball “out” when it is “in” than vice versa.
The mistake arises because of the way a person’s eyes and brain interpret moving objects. Scientists already know that the people can make mistakes about an objects position, depending on its motion and the motion of other objects in the area. The team decided to look for a real-world example of this phenomena, and settled on tennis due to a new rule allowing players to challenge a referees decision.
If a player makes a challenge and the referee is found to be incorrect, the player is allowed to make further challenges. If the ref was right all along, however, the player is no longer permitted to question their judgement. This means that knowing when a referee is more likely to have made a mistake can give a player the advantage.
There are two kinds of mistake a tennis referee can make. As the ball flies towards the edge of the court, the referee must carefully observe where it lands. If it’s behind or on the line – even by a tiny fraction – then the ball counts as in. When the referee judges a ball to be out when it was actually in, this is known as a “predicted” error, whilst a ball judged to be in when actually it was out is called an “unpredicted” error.
If a referee was completely unbiased, we would expect them to make both kinds of errors equally. After all they’re only human, and a few mistakes here and there are not to be unexpected. By analysing 4,457 tennis points randomly drawn from Wimbledon 2007 the scientists discovered 83 referee errors, and observed that 70 of the errors reported the ball in being shifted in the direction of the ball’s motion, a predicted error. In other words, referees are calling the ball out instead of in much more than the other way round.
This suggests that players should challenge close-calls as often as possible, because the referee is more likely to have made a mistake. On the other hand, if a player believes the ref has erroneously called a ball in then they should keep quiet – because there’s only a small chance that they’re right! The authors suggest that ideally every shot in a tennis match should be objectively reviewed by instant replay – but given that this is unlikely, the adoption of clay courts (on which balls leave marks) as used by the French could solve the problem all together.