Advanced metrics: Expected Goals
As part of Opta’s commitment to offering greater detail to sports fans across the world, they have been working on some advanced metrics that help explain the game in more detail.
One of these metrics is Opta’s expected goals model. Based on over 300,000+ shots, expected goals (xG) measures the quality of a shot based on a number of different variables. The metric gives an indication as to how many goals a player or team should have scored on average based on the chances they have had. See the explainer video here:
Below you can see the expected goals leaderboard from the 2016/2017 season, where both Harry Kane and Romelu Lukaku had particularly good goal-scoring seasons. Their respective xG totals of 13.86 and 14.53 show that they were particularly lethal in front of goal and were often taking chances with a low likelihood of scoring.
With Opta’s expected goal metric you can also visualise the chances that a given player has during the course of a game or a season. As you can see from Harry Kane’s shotmap below, the striker was finishing the chances with a high expected goals value but also scoring goals where the probability of them being scored is low.
The expected goals metric can also give an indication of a team’s underlying performance throughout the season. Below is Crystal Palace’s expected goal difference in comparison to their actual goal difference. Sam Allardyce replaced Alan Pardew for matchday 18 withPalace lying in a precarious 17th place and failed to win any of his opening 5 games.
Palace’s expected goal difference was telling a different story though and shows that whilst they didn’t get any points from these games they were showing signs of an improving team. With a higher expected goal difference than their actual goal difference, Palace were demonstrating that they were in fact scoring less and conceding more than Opta’s expected goals model was showing meaning that an upturn in their fortunes was due.
Their first win under Allardyce came in a 2-0 away victory against Bournemouth on matchday 23 and from then onwards they dropped only 11 points from a possible 33 to secure their Premier League survival.