A Challenge System for the “Cheating”

Two teenage boys are playing a practice match. One serves an ace out wide and does a little fist pump, while the other unhesitating, calls it out. “Are you sure?” asks the server. “Positive” replies the returner. In a match, this would be an altercation and a roving umpire would be called for following points, based on suspicion of playing a “cheater”, bad eyes or not. But its practice, the smirking server yells out, “I demand a challenge!” As if! Challenges are for the big time only.

“Cheating” is an unspoken truth in the game of tennis. Tennis is a fast sport that requires rapid and accurate perception of the line to call the landing of the balls, and the ability to make a split second decision. Not everyone is perceptually accurate unfortunately, and not everyone has perfect vision and reaction, not recreational players’ young and old, hungry for each win juniors and not even professional tennis linesmen. There are many contributing factors other than sight, such as concentration, gamesmanship and bias. These cause many disputes among the tennis fanatics, ranging from those enjoying a hit in the park; bickering parents watching their kids play local tournaments, juniors trying to get an edge in order to secure a win, and even pros questioning the refs and refs overruling other refs. That goes both ways as well; some players miss calls and rule against their own favor. How about that match at Wimbledon when the umpire messed up the score against Venus and nobody noticed until the match was over? But that hardly ever causes controversy, it is always more interesting to follow the drama. Needless to say that some of today’s professional greats have been notorious cheaters in the juniors, but we won’t name names here. We will just take a look into the modern day savior, of course technology, of course line-call challenges.

George Mather, a psychologist in the UK, published an interesting study called “Perceptual uncertainty and line-call challenges in professional tennis” in the Proceedings of the Royal Society journal in April of this year. The challenge technology now provides physical evidence that can be compared to the opinions of players and line judges. Human observation has undergone many tests throughout time and while it as been proved that humans can be extremely precise in their perceptual judgment, our vision is still limited, because our eyes don’t allow us 3D sight spanning 360 degrees around the object we are observing, thus, there will always be uncertainty. There is also some internal noise and our reactions to similar physical situations never produce exactly the same neural responses, which causes us to hesitate in our uncertainty.

What is the challenge system? The ATP uses the Hawk-eye ball tracking system which can locate 3D position of the ball within 3mm of the bounce. Multiple cameras are setup on the court and image processing software follows the trajectory of the ball from all its sides in each video frame. Players are the allotted no more than two incorrect challenges per set, and the balls are still called by linesmen, not to take away their jobs entirely. Since all guesses are recorded and so are all the challenges and their outcomes that is a lot of data that can reveal some interesting things. To be precise, this study evaluated 1473 challenges made by 246 pros during 15 ATP tournaments worldwide in 06 and 07. Keep in mind that 100mm is less than twice the diameter of the ball. What they found was that calls are challenged 94% of the time when the balls bounce within 100mm of the line, which makes sense; it’s harder to make close calls. The other 6% occur over a wide range of bounces greater that 100mm. The greatest distance recorded was 449mm and the line judge called that one easily and correctly. The challenges are made equally for balls called out and balls called in, which shows that no matter how far the player is from the ball that is not a cause for more or less challenges. They found that 60% of the calls challenged were correct, a positive statistic towards human vision and perception and a plus towards the rule allowing only 2 bad challenges per set. However the remaining 40% were incorrect and that is of significant importance. To think before the hawk eye, 40% points were lost unjustifiably. Ball bounces along the base and service line are evidently much harder to judge than side lines, this is due to an uncomfortable visual trajectory and speed of the ball.

This particular study implemented a statistical model which gave a clear reading of human accuracy. It is also important to remember that even if professional tennis players exhibit proficiency to just a few centimeters of the bounce, they have been staring at tennis ball landings for years, and their eyes are thus more trained, as are the line judges. This is clearly not the case for recreational and junior tennis players. The model of this study hypothesized that all balls called within 100mm of the line would be called incorrectly due to limitations of their vision and perception. They also figured that an even pro match consists of 50 points per set, or 10 games made up of 5 points each, with each point getting a call. If errors are made 40% of the time, then the expected number of errors in this model is 4 per set. Of course, not every point ends in a borderline call, and that can be logically reduced to 2 errors per set.

For our purposes this study shows us that human error is inevitable when doing their best job in professional tennis. Most likely, in junior tennis, the untrained eye and the lack of refereeing on each court, as well as temptation for gamesmanship is still a problem. And it shows us that tennis deserves this technology because a lot of sweat, pride and hard earned money are on the line. Especially in a sport as mentally difficult, where one point lost may not take a tow on physical stamina, but may break someone mentally. The next thing is, imagining a world of possibility for fair play. What if, one day, the challenge system will be in effect at the park, at the local junior tournaments as well as all the professional events? Maybe that will change the game, maybe it is about time it should.