ENS

London, Apr 17: "You can NOT be serious," tennis great John McEnroe famously shouted in Wimbledon in 1981, when one of his serves was called out. McEnroe's words became a catchphrase to bait umpires and line judges around the world - but more than a quarter-century later, match officials have found an answer in science. A paper published on Wednesday in British journal, Proceedings of the Royal Society B, has analysed 1,473 challenges to line calls by 246 professional tennis players in 2006 and 2007.

The study compares the line judge's call and the player's challenge with the final word from Hawk-Eye - a hi-tech ball-tracking system used by the Association of Tennis Professionals (ATP) that can spot the position of a ball in play to within three millimetres (0.12 of an inch). Professional players and line judges "are remarkably proficient" at judging ball bounce position, displaying an accuracy to within just a few centimetres (a couple of inches), when the ball is travelling at 50 metres per second (180 kilometres, 112 miles per hour), says author George Mather, a University of Sussex psychologist.

But the line judges were more reliable than the players. According to Mather's calculations, the judge is right 61 per cent of the time when challenged.

Mather found most erroneous calls happened, when a ball bounced within 100mm (3.9 inches) of a court line. Nearly one in 12 of such events were wrongly called by line judges. And, says Mather, these miscalls were far likelier to happen when the ball bounced near the base and service lines rather than near the side and centre lines.

One reason for this could be that base/service line judges sit much closer to the line than the side/centre judges - a distance of around 5.5m (17.8 feet) as opposed to 8.7m (28.25 feet). The image of the ball may pass too fast across the retina for the brain to get a precise fix on the ball's position, a phenomenon called retinal motion bias.

The bias may affect balls struck down the centre of the court rather than diagonally Such balls travel .

much faster across the judge's field of view. "Training and line judge selection should focus on maximising performance for base and service line calls, since these are the most error-prone," the paper says.