What is xG? - Football's Strangest New Philosophy [Learn How to Use It In Betting]

 

There are many things in football worth getting genuinely worried about: sub-par officiating, whether that involves VAR or otherwise; your team’s games repeatedly being re-arranged for TV purposes at huge inconvenience to match-going fans; the foul-mouthed, non-stop nonsense-spouter sat behind you in the stands. All of these impinge upon one’s enjoyment of the game because they’re so inescapable. That’s something which doesn’t – or certainly shouldn’t – fall into the same bracket as xpected Goals (xG).

Expected goals, or xG, have become a stick with which to beat the modern game - but resisting it is strange

What's Expected Goal or xG? 

When referring to football, the term xG stands for 'expected goals'. Simply put, xG is a metric that calculates the probability of a certain shot resulting in a goal. 

The probability that a shot will result in a goal based on the characteristics of that shot and the events leading up to it. Some of these characteristics/variables include:

Location of shooter: How far was it from the goal and at what angle on the pitch?

Body part: Was it a header or off the shooter's foot?

Type of pass: Was it from a through ball, cross, set piece, etc?

Type of attack: Was it from an established possession? Was it off a rebound? Did the defense have time to get in position? Did it follow a dribble?

Every shot is compared to thousands of shots with similar characteristics to determine the probability that this shot will result in a goal. That probability is the expected goal total. An xG of 0 is a certain miss, while an xG of 1 is a certain goal. An xG of .5 would indicate that if identical shots were attempted 10 times, 5 would be expected to result in a goal.

There are a number of xG models that use similar techniques and variables, which attempt to reach the same conclusion. The model that FBref uses is provided by StatsBomb. What sets StatsBomb's xG model apart from others is their use of freeze frames.

 A freeze frame is the location of all players on the pitch at the moment the shot was taken. Was the goalkeeper in position? Was it an open goal or were there a number of defenders between the shooter and the goal? Was the shooter being pressured? Was it a 1v1 situation with the keeper?

Take this Callum Wilson goal vs Southampton for example. The shot was taken directly in front of the goal from six yards out. However, Wilson was the only player in the penalty area at the time of the shot, making it a completely open goal. According to StatsBomb's data, just 3% of shots from this location were taken with an open goal. Comparing this shot to all other shots taken from this spot without accounting for the location of the defense would return a wide range and inaccurate set of results. In fact, other expected goal models credit this exact shot anywhere from 0.5 to 0.66 xG. StatsBomb, and their use of freeze frames, credits this shot with .97 xg, making it an almost guaranteed goal.

xG does not take into account the quality of player(s) involved in a particular play. It is an estimate of how the average player or team would perform in a similar situation.

How xG is used

xG has many uses. Some examples are:

Comparing xG to actual goals scored can indicate a player's shooting ability or luck. A player who consistently scores more goals than their total xG probably has an above average shooting/finishing ability.

A team's  xG difference (xG minus xG allowed) can indicate how a team should be performing. A negative goal difference but a positive xG difference might indicate a team has experienced poor luck or has below average finishing ability.

xG can be used to assess a team's abilities in various situations, such as open play, from a free kick, corner kick, etc. For example, a team that has allowed more goals from free kicks than their xGA from free kicks is probably below average at defending these set pieces.

A team's xGA (xG allowed) can indicate a team's ability to prevent scoring chances. A team that limits their opponent's shots and more importantly, limits their ability to take high probability shots will have a lower xGA.

Penalty Kicks

Each penalty kick is worth .76 xG since all penalty kicks share the same characteristics. Comparing a player's goals from penalty kicks to their penalty kick xG can indicate a player's penalty kicking ability. Likewise, we can do the same for goalkeepers in these situations.

FBref's xG totals include penalty kicks unless otherwise noted. For xG excluding PK, we recommending using npxG (non-penalty expected goals).

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How we calculate xG totals for a single offensive possession

In some cases, a player or team's xG totals do not equal the sum of their shots. For instance, a team may attempt multiple shots in a single possession, but it is likely that these shots are contingent upon the outcome of the previous shot(s).

Take for example, this match between Schalke 04 and Nürnberg:

 In the 78th minute, Nürnberg attempted three shots which ultimately led to a goal. Hanno Behrens attempts a shot that is saved, but he is able to take a second shot as the ball is deflected off the defender. The second shot goes off the woodwork, which allows Adam Zreľák to easily tap it in. According to StatsBomb's expected goals model:

Behrens' first shot with the goalkeeper in his way = .37 xG

Behrens' second shot with the goalkeeper out of position but a defender in the way = .68 xG

Zreľák's shot with an open net = .81 xG

The sum of these three shots is 1.86 expected goals, even though it is impossible to score more than one goal in a single move. To solve this problem, we find the probability that the defending team does not allow a goal in this possession. In this case, the calculation is:

(1 - .37) x (1 - .68) x (1 - .81) = .0383 or a 3.83% probability that Schalke does not allow a goal.

To find Nürnberg's xG, we simply subtract that probability from 1:

1 - .0383 = .9617 xG

In other words, we estimate that an average team in a similar situation would be expected to score a goal 96.17% of the time.

We use a similar method when calculating xG for individual players. Adam Zreľák receives .81 xG from his single shot while Hanno Behrens receives:

1 - (1 - .37) x (1 - .68) = .7984 xG

This shows why a team or player's total xG may not equal the sum of the xG from their shots and why a team's total xG may not equal the sum of the xG from their players.

Possessions that include a penalty kick

Similarly, we include shots taken from a rebound after a penalty kick with xG from penalty kicks. Take this Alexis Sanchez penalty kick for example:

As mentioned above, the penalty kick attempt = .76 xG

The second shot after the rebound, from 6 yards and with the goalkeeper unrecovered from the save = .72 xG

Since the second shot is a result of the first, we use the same probabilistic method in the previous example. Rather than a total 1.48 xG (.76 + .72), the calculation is:

1 - (1 - .76) * (1 - .72) = .9328 expected goals

However, since the second shot is also considered to be a part of the penalty kick xG, Sanchez gets 0 npxG (non-penalty expected goals) on this play.

Note: We treat corner kicks and free kicks as a new possession, not a continuation of the previous possession, but are continuing to study the issue.

On July 31, 2020, StatsBomb upgraded their xG model with the inclusion of shot impact height, which is the height of the ball when a shot is struck. For many shots in which we know the height of the pass preceding the shot, there will be little to no impact. For the other shots, however, there is a "sizeable impact" on their xG value. This update improves the quality of an already industry-leading xG model.

What is Post-Shot xG (PSxG)?

 xG, or what can be considered "Pre-Shot xG", is calculated considering all shots at the time of the shot without knowing the quality of the shot attempt. It not only includes shots that are on target, but also shots that are deflected or off target. Post-Shot xG is calculated after the shot has been taken, once it is known that the shot is on-target, taking into account the quality of the shot. As with xG, PSxG is provided by StatsBomb and is further explained here.

All shots which are off target will have a PSxG of zero since there is a 0% chance that this trajectory will lead to a goal. For shots that are on target, their PSxG will almost always be higher than its xG value since it is a better than average shot compared to all shots with similar pre-shot characteristics.

When evaluating a goalkeeper's shot stopping ability, we only want to include shots that are on target since these are the shots where the goalkeeper can have an impact. Therefore, we use PSxG to estimate the quality of shots in which they have faced.

What is xA?

xA, or expected assists, is the xG which follows a pass that assists a shot. This indicates a player's ability to set up scoring chances without having to rely on the actual result of the shot or the shooter's luck/ability. Note: Because xA comes from passes, not all assists will be given an xA value.

Where to find xG

Team xG, xG against, and xG difference can be found on league tables, such as this:

Playing Time:

Performance Per 90 Minutes

Expected Per 90 Minutes

Expected goals can also be found on a number of different pages such as league player stats, match reports, player pages and player match logs.