Blog of Random Thoughts and Pictures

Expected Goals (xG)

May 9th, 2021

As part of my LOI weekly predictions (don’t ask about match day 10 it was a wipe out) I’ve also included an indication of the teams expected goals (xG). Now it might be worth articulating an introduction to expected goals and there are four videos worth a review.

  • One by David Sumpter (Friends of Tracking)
  • One by Duncan Alexander at Opta.
  • One by Tifo football.
  • One with example goals with the xG overlaid on the screen.

First up David Sumpter on How to explain expected goals to a football player.

In this video David takes us through the probability of scoring a goal in the penalty area, with an overview of Barcelona statistics of expected Goals, indicating how a penalty is a 75% chance of scoring, and in comparison to a 7% chance of scoring, and an explanation of what this means.

True to the point there can and should be a reasoned discussion around goal scoring (and goal prevention) instead of always the emotional one, and xG gives some insights on this.

Opta’s Duncan Alexander takes us through the expected goals metric in the video Opta Expected Goals.

Of note (agt the time of the video), 4 variables are considered with Opta, Assist Type, Header / Foot, Big Chance and Angle/Distance.

The video by Tifo Football is a nice By The Numbers presentation on What is xG ?

They describe in nicer detail how good a shooting chance was, how likely a similar chances was, to result in a goal. They also highlight that people like StrataBet considers defenders in the way while other models like Opta do not and that sets of 5~10 games get best value for xG.

Expected goals (xG)

So expected goals (xG) is a probability of scoring a goal, with a look at how good a shooting chance was, how likely a similar chances was, to result in a goal.

Finally here’s a video demonstration of expected goals (xG)