What is the error margin of goal line technology?

During the Premier League match between Aston Villa and Fulham on April 5, Lewis Holtby had a shot which was cleared off the line and adjudicated on by the goal line technology used by the league. Plenty of opinions were available on Twitter in the aftermath of this incident as it was so tight but none focused on the most important aspect, namely how accurate is the system?

Firstly, take a look at Lewis Holtby’s shot, which can be found at around the 3:30 mark on the video below.

 

After this happened, Al Jazeera presenter Richard Keys tweeted the following:
Keys

The suggestion from Keys is therefore that his own eyes are better at identifying whether a goal had been scored than the Hawkeye technology used by the Premier League. In order to assess this, we need to know two things; the error margin of the  Hawkeye system and the error margin of Keys’ eyes in these situations. Hawkeye seems to claim 5mm but I have been unable to find an independent assessment, only claims from Hawkeye themselves. As for Richard Keys’  eyes, we will have to make do with the human eye and I am hoping that readers might be able to point me in the right direction on this as a quick google search hasn’t provided me with an answer.

 

Soon after Keys’ tweet came another from Sporting Intelligence’s Nick Harris:

GLT

The problem here is that it is presented as if Hawkeye is 100% correct which we know is not the case given the company’s own literature on its accuracy. The image which ‘proves’ it isn’t a goal is a computer generated image from the Hawkeye system but how accurate is that system? If the ball was over the line by less than 5mm, the system could be wrong if the company’s own claim of a 5mm error margin is correct.

 

The two media opinions on Hawkeye’s decision, and others like them from twitter are surprising in the main because they either put 100% faith in the human eye or 100% faith in the technology. Both though have error margins and in the end, the choice of goal line technology above anything else should be made on one simple test – is the system being used better than all of the other alternatives?

 

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15 Responses to What is the error margin of goal line technology?

  1. Lars says:

    Complex topic. I just want to add one thought:

    As you might remeber from slo-mo tennis videos but also football, the ball is not perfectly round when it is about to be hit by someone. Instead, it is quite drastically deformed, more than you would expect.

    The Hawkeye “proof” does not take that into account, it looks perfectly round there.

  2. Dave Smith says:

    The answer is an emphatic “yes”. Human perception doesn’t come close to this sort of accuracy, as had been proven time and time again over the years.

  3. Aaron says:

    We’ll never know the “true” decision here. It might actually depend on just how well the line was painted. I’d still rather go with Hawkeye’s opinion than any individual’s.

    But just as in tennis/cricket, Hawkeye’s biggest benefit isn’t actually in getting things correct [though it needs to be 99.9%+ on that], it’s in being consistent and unbiased. If Wayne Rooney had ‘scored’ in a similar fashion to Holtby in front of the Stretford End, Hawkeye would make the same decision.

  4. Sacha says:

    You can read about how the system is tested here http://www.fifa.com/aboutfifa/news/newsid=2310048/index.html how the ball deforms when it hits an object is part of the testing, the graphical representation of it on screen is always going to look a bit different.

    Hawkeye claim its 3mm, but as with a ref, the system should be 100% certain of a goal, so it should err on the side of caution. It shouldn’t distract from the fact that Holtby should have buried the chance

  5. Sacha says:

    Should have also added this to the post, a link to Labosport explaining the testing, they did the tests on all 21 systems running in England at the moment http://quality.fifa.com/en/News/Goal-line-technology-set-up-ahead-of-FIFA-World-Cup/

  6. Are there published accuracy comparisons available anywhere?

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  8. Robert says:

    What isn’t mentioned is that we should compare the Hawkeye system and the split second decision the referee (team) needs to make. Based on how tight this is, I believe the referee (or his goal line assistant even) can be 100% sure that this is a goal, so therefore they should not give the goal. With the speed of the ball, the post in the way and the pressure of some 30.000 or more people in the stadium I don’t know how we have ever expected the refs to make the right call (which is to allow a goal only if they are 100% sure it is in).
    Hawkeye may be wrong is very few cases, but at least it will be consistently so.

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