When fumed, 鈥測ou cannot be serious,鈥� he might have been onto something. The visual limits of tennis line judges mean they鈥檒l flub almost a tenth of all close calls.
鈥淎 certain number of errors are inevitable,鈥� says , a psychologist at the University of Sussex, UK, who analysed wrong calls in professional tennis tournaments with a mathematical model of human perception. 鈥淓ven the best line judges are always going to make a few errors,鈥� he says.
Professional players faired just as poorly in judging calls. Or perhaps that should be just as well, considering that tennis balls can blast across the court at well over 50 metres per second.
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Overuled
For most of tennis鈥� history a contentious line call has generated a bemused look, an occasional tantrum and, in rare cases, a player鈥檚 disqualification. A line judge鈥檚 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 . 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 line judges remain the workhorses of tennis officiating. Players must challenge a call to see the Hawk-Eye鈥檚 decision, and they鈥檙e allowed only two unsuccessful challenges per set.
Mather, a psychologist interested in perception, though not a tennis player (鈥淚鈥檓 more of football fan,鈥� he says), saw the calls as a perfect test of how well people judge moving objects in real life. 鈥淚t occurred to me while watching those tournaments that they were doing an experiment for me.鈥�
Not always right
Mather analysed data from 15 men鈥檚 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. When Roger Federer smashes a forehand winner that lands a few millimetres from the sideline, a judge鈥檚 brain does its best to pinpoint the ball.
鈥淲hen you鈥檙e reaching a decision about exactly where a ball is on the court, your brain will not always come up with the same answer,鈥� Mather says.
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 estimates.
Trust your instincts
Mather鈥檚 findings give real-world confirmation to lab experimentation, says , a behavioural neuroscientist at the University of Texas Medical School at Houston. 鈥淵ou can鈥檛 bring an fMRI [brain scanner] into the field to figure out how these decisions are being made,鈥� he says.
Professional tennis players ought to trust their instincts when they think they鈥檝e been wronged by a line judge, Mather says. 鈥淚t鈥檚 in the players interest to mount some challenges, there鈥檚 bound to be a few errors taking place,鈥� he says.
Still, some players might be more interested in picking a fight than winning a point. 鈥淵ou don鈥檛 know from looking at the old pictures of John McEnroe whether he thought the ball was in or out, or whether he was doing it for some other reason,鈥� he says.
Journal reference: (DOI: 10.1098/rspb.2008.0211)
