This afternoon, I tuned in for the last two games of the Wimbledon semifinal between Andy Murray and Jo-Willy Tsonga. Andy Murray won the match, making him the first British male finalist at Wimbledon in 74 years (no pressure, Andy!).
The winning point was a bit strange, however. Murray hit an aggressive shot that was a tough call for the umpire. The crowd exploded, but then quickly realized the ump called it out. No matter. Murray put a finger in the air and challenged the shot. Under tennis replay rules, players can challenge in or out calls, and the disputes are decided by the Hawk-Eye ball tracking system. Hawk-Eye uses high frame rate cameras to map the trajectory of the ball. The Hawk-Eye trajectory showed the ball nicking the outside of the line, and Murray was through to the finals!
I asked my buddy Tony, a staunch replay opponent, whether he would have felt bad if Murray had lost the match in the absence of replay review. He said no, because he guessed the tennis replay system faces precision issues. This is his standard “blades of grass” complaint. On extremely close plays, there is no way to tell whether a ball (or player, in other sports) is in or out, because it comes down to the deformation of the ball on impact, individual blades of grass, and lack of precision in painted lines.
I set about to prove him wrong, under the premise that a computer-driven system like Hawk-Eye would have definable precision. In one way, I was right. Hawk-Eye’s average error is 3.6 mm. But if that’s the case, why is it used to make calls that are well within it’s margin of error? Once a challenge is made, it doesn’t matter how close the ball is; Hawk-Eye makes a final decision one way or the other. Murray’s final shot wasn’t in by much more than 3 mm, and now I wonder if we should feel bad for Tsonga.
The tennis situation mirrors replay issues in many other sports. I’m a strong replay review supporter, but I think that “indisputable evidence” (or “incontrovertible,” or whatever) should be an important part of any replay system. Replay should be used to avoid big mistakes, not decide impossible calls. The challenge, then, is not going overboard as in tennis. Tony’s argument is that we will always go overboard and start deciding indeterminate calls on weak evidence.
There’s a potential solution for tennis (and soccer, as it adopts goal line review technology). Right now, Hawk-Eye shows only the most likely location of the ball. Instead, it should show the most likely location of the ball with a reasonable error range shaded around it — the area that Hawk-Eye projects the ball landed in with 99% certainty, or 99.9% certainty, for example. If the error range is too big to make the call, the original call should stand.
Note: I forgot to mention, Adrian the Canadian has discussed the Hawk-Eye technology on this blog before, albeit from a different angle — it’s use in cricket. In cricket, it is used for a judgment call (Leg Before Wicket). Replay review usually steers clear of judgment calls, but the Hawk-Eye seems to be going over very well.