Expected Goals (xG) aren’t new but they can still be a bit of a mystery.
To help spread understanding of what goes into grading these chances I like to present the different factors that feed into the rating of each shot taken. Also it is an international break week, following a weekend where Arsenal didn’t play, us content creators have got to do something to fill our time with.
Without further adieu here are the top 5 chances from match week 31 of the Premier League.
5. Terence Kongolo, Philip Billing and Jason Puncheon
Ok so I’m already breaking my own arbitrary rules here and giving three shots for the 5th spot. The reason that I am doing this is to not only point out that Kongolo’s shot was one of the 5 highest but to also help answer the question of how multiple shots that are all part of one sequence are handled.
This shot was saved and fell to Philip Billing:
This shot was not hit clean and then fell to Jason Puncheon:
Overall, that is 3 shots, with a total xG of 1.43. Yes I know 1.43 is larger than 1, which represents 1 goal, so how is that possible and why didn’t a goal get scored.
Well I am glad you asked. The way that I handle situations like this is by using conditional probability for shots that are part of the same sequence. For this, a new sequence starts when the ball goes out of play, the defensive team takes possession of the ball and completes 3+ offensive actions, or more than 15 seconds elapse between shots (this is a made up cutoff but I feel like after more than 15 seconds a defense should be able to regroup and become set again).
For these 3 shots, they are all part of the same attacking sequence and the xG for the full sequence breaks down like this:
The first shot is treated normally, the second shot depends on the probability of the first shot not being converted into a goal (0.508) and that is multiplied by the xG of that shot and gets you 0.232. The third shot is dependent on both the first and second shots not being goals and that has a probability of 0.276, so you then multiple that by the xG of 0.48 to get an xG for that shot of 0.133. Overall there is an xG of 0.857 for the sequence of 3 shots.
4. Gonzalo Higuaín
This chance was created by Jorginho playing a gorgeous first time through ball over the top of the Everton defense.
Don’t believe the haters who say he only passes sideways Jorginho is bad and Chelsea should look to get rid of him this summer, he doesn’t provide anything for their team besides sterile possession.
Thankfully for Arsenal’s top 4 odds Higuaín didn’t finish this shot and Chelsea went on to lose.
3. Mohamed Salah
This was a great two man counter attack chance by Liverpool. It started with Salah coming back to pressure the ball. Sadio Mané getting to the ball first and then both running like crazy to attack. One of the more interesting parts of this chance was the run taken by Salah that really seemed to confuse Fulham.
Instead of running down the right channel, Salah runs behind Mané and that leaves Arsenal loanee Calum Chambers in the nearly impossible position of having to defend both players. In the end Salah’s shot is saved but it doesn’t change the result for either team.
2. Sadio Mané
This shot is the result of a one-two with Roberto Firmino. It ends with the ball cut back to Mané to slot home. It is something that Unai Emery would love.
1 Ryan Babel
This is a situation where xG really underrates the quality of the chance due to a lack of information in the event data.
In the lead up to this shot, James Milner slices a clearance, Virgil van Dijk under hits his back pass header and then Babel wins a 50/50 with a fortunate bounce that leads him all alone with an empty net to shoot at.
It is still rated highly but this should be as close to 1 as xG comes.