“This assigns each shot a probability related to its position, and thus determines how well a goalscorer is performing. The statistic filters out the quality of the opposition and the quality of the player's team. Last year, for instance, Tottenham's Gareth Bale had 161 shots and 21 goals, when, according to the goal-expectation model, he was due to score only 11. "Bale would regularly shoot from situations with a low probability of success, such as from a distance of 30 yards, and score," says Paul Boanas, Prozone's senior account manager and a former performance analyst. "This type of contextual information helps to explain why he's worth so much."
Some of the most important elements of football remain very hard to quantify and it's difficult to understand what we can't measure. Consider defence. Using data from the last ten seasons of the Premier League, Anderson and Sally compared the value of a goal scored and the value of a goal conceded. They found that scoring a goal, on average, is worth slightly more than one point, whereas not conceding produces, on average, 2.5 points per match. "Goals that don't happen are more valuable than goals that do happen," Anderson says. "It's counterintuitive. The question is: how do we measure something that doesn't happen? The challenge is to see the unseen."
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