Monday Night Football Extra: Expected goals explained
Last Updated: 17/08/17 1:28pm
In this week’s Monday Night Football Extra, David Jones and Jamie Carragher examine the use of ‘expected goals’ and why the model can be a useful tool to help explain the game…
Did Chelsea deserve to beat Burnley on the opening weekend? Could Crystal Palace actually have had the better of it in their 3-0 defeat to Huddersfield? Few were queuing up to say so but Opta's 'expected goals' model tells us a different story of those matches.
Monday Night Football has long been at the forefront of in-depth football analysis so it will be no surprise that the subject of 'expected goals' has been discussed for some time behind the scenes. But what exactly is this measure and how can it be used to help our understanding of the game?
"Primarily, expected goals are a measure of chance quality," explains host David Jones.
"Our partners at Opta have analysed over 300,000 shots so they are able to calculate the likelihood of a chance being scored from a particular position on the pitch and in a particular phase of play.
"It takes into account factors such as the distance the shot was taken from, the angle, the type of chance - so was it taken with the foot or with the head - and the assist type as well."
It is customary in match analysis for teams to be compared in terms of their number of shots. But all attempts are not equal. Expected goals (xG) puts a value on each shot and in doing so provides a more detailed and accurate representation of a team's chances to score.
For example, Christian Eriksen's long shot for Tottenham against Newcastle on Super Sunday at the weekend had only a two per cent chance of being a goal, with an xG rating of 0.02 based on Opta's expected goals model. "It's basically one in 50, which is probably why managers don't like players shooting from that range," says Jamie Carragher.
"We should remember that individual players are going to be slightly different. So a player of the quality of Christian Eriksen you would expect to be slightly higher than that. But he is not going to score 10 or 12 from that range in 50 shots. Less technical players like myself would be less than two per cent, but it shows how difficult it is to score from that range."
In contrast, Hector Bellerin's second-half chance in the right channel during the second half of Arsenal's 4-3 win over Leicester on Friday Night Football, a moment easily overlooked in a hectic contest, was a far better opportunity - rated as a 33 per cent chance of being a goal.
The cumulative totals of these chances can reveal stories that challenge our perceptions about a game. For example, while Chelsea and Palace suffered miserable home defeats on the opening weekend of the Premier League season, neither result reflected the quality of chances that those sides enjoyed.
Chelsea's expected goals total was actually greater than Burnley's despite losing 3-2 at Stamford Bridge. In part, this is because Stephen Ward's spectacular effort that doubled the visitors' lead was not actually a clear-cut scoring opportunity at all.
Crystal Palace also had the greater chances in their 3-0 defeat to Huddersfield, while Southampton's expected goals total of 2.02 suggests that they created enough opportunities to see off Swansea despite the game finishing goalless.
"The more data that we build up through the season, the more interesting the stories around expected goals that we should be able to bring you on Monday Night Football," adds Jones.