Memo to tennis players: because of the way the human visual system works, referees are more likely to call “out” a ball that actually lands in, rather than call “in” a ball that in fact lands outside the line. Now that professional players are permitted to challenge calls, therefore, they would do well to focus on balls that are called “out,” since they are more likely to be wrong.
So concludes a neat little study published online today in Current Biology. Scientists led by David Whitney of the University of California, Davis, started from the fact that the human visual system consistently misperceives moving objects as shifted in the direction of their motion, making them appear to be farther along their path than they are.
To see what effect this might have in a tennis match, Whitney and his colleagues analyzed 4,457 randomly-selected points from the 2007 Wimbledon tournament, focusing on those where the ball landed close to the line. Using video from the matches, they uncovered 83 incorrect calls. Of those, 70 were balls that were called out when they actually fell inside the line. Only 13 were called in when they were actually out. Prediction confirmed: the brain thinks a moving object is farther along in its path that it really is.
Under the new rules for challenges, players get two (in the U.S. and Australian Opens) or three (at Wimbledon) incorrect challenges per set. Given the bias in incorrect calls, it is clearly to a player’s advantage to focus on balls that are called “out” in their opponents’ court rather than balls called “in” on their own side of the net: the former challenges are more likely to be upheld, while the latter count against the two or three incorrect challenges a player is allowed. As long as a player continues to challenge incorrect referee calls, he or she is allowed to continue making challenges. (At Wimbledon in 2007, there were about 140 challenges, resulting in 25% of the calls being overturned.) Players can improve their odds of challenging only incorrect calls if they focus on balls called “out.”