London, April 16 : A new analysis has suggested that the visual limits of tennis line judges mean that they are bound to mess up almost a tenth of all close calls.
According to a report in New Scientist, the analysis was done by George Mather, a psychologist at the University of Sussex, UK, who analysed wrong calls in professional tennis tournaments with a mathematical model of human perception.
"A certain number of errors are inevitable," said Mather. "Even the best line judges are always going to make a few errors," he added.
For most of tennis' history, a contentious line call has generated a bemused look, an occasional tantrum and, in rare cases, a player's disqualification.
A line judge's call used to stand, unless the umpire was able to get a better look or the ball left a clear smudge on a clay court surface.
That has all changed with the advent of a new line-calling system dubbed Hawk-Eye.
Several high-speed cameras capture every bounce and pinpoint a ball to within 3 millimetres to enable it to make a clear in or out call.
But, players must challenge a call to see the Hawk-Eye's decision, and they're allowed only two unsuccessful challenges per set.
Mather saw the calls as a perfect test of how well people judge moving objects in real life. "It occurred to me while watching those tournaments that they were doing an experiment for me," he said.
He analysed data from 15 men's professional tournaments, which included 1473 challenges made by 246 players.
Unsurprisingly, players and linesmen were most likely to misjudge balls landing within 5 millimetres of the line. The farther a ball landed from the line, the fewer the mistakes.
To explain this pattern, Mather created a mathematical model of line calling based on human perception of fast-moving objects.
According to Mather, "When you're reaching a decision about exactly where a ball is on the court, your brain will not always come up with the same answer." Because of this ambiguity, line judges will still misjudge 8.2% of all balls that land within 10 centimetres of the line. This could add up to four botched calls in a fairly close set, Mather estimated.
This uncertainty makes it important for professional tennis players to trust their instincts when they think a line judge has wronged them.
"It's in the players interest to mount some challenges, there's bound to be a few errors taking place," said Mathers.