As new terminology goes, nothing seems to confuse people more than the expected goals or xG metric. Simply put, expected goals measures the overall quality of a particular chance, which is calculated by how likely that chance will end in a goal. xG is based on a number of factors before the shot was taken.
The scale on which xG is rated is from 0-1. As you would expect, zero represents a chance that is impossible to score, while one will represent a chance that a player is expected to score every single time. You may see match statistics showing that Arsenal have an xG of 2.3 and Tottenham have an xG of 1.92. This would allude to the fact that the Gunners have had a larger likelihood of scoring than Spurs in the game.
It is worth bearing in mind that this metric does not show who should win the game, with xG only measuring the quality of chances. This comes down to the varying factors that affect xG, with these including pattern of play, distance from goal, one-on-one chance, the angle, big chance, type of assist and the specific body part.
How Is xG Worked Out?
The expected goals metric is something that some fans feel is a needlessly convoluted inclusion in any game, with it just breeding confusion. After all, Arsenal may boast an xG of 2.3 in their game against Spurs, but from one game to the next it is very easy to underperform or overperform in comparison to that number if it had stayed the same.
Of course, it will likely change from game to game, but it is worth bearing in mind that a team could have an xG of 1.65 and then score three times, while they could have an xG or 2.5 the following week and fail to score a single goal. As mentioned above, the variables that are taken into account and used to work out how likely a chance was to end in a goal are:
• Distance to the Goal
• Angle of the Effort
• Body Part that Took the Shot (eg. strong foot, weak foot, header)
• Passage of Play
• The Shot
• Chance Creation
• Defensive Play
This means that if a player has the ball rolled to them and all they have to do is poke the ball into an unguarded net from a few metres out, then the chance will be close to one. This chance can be made less likely if a defender is near to them, the pass is poor, the player taking the shot is at an angle or they have to take it on with their weak foot.
All of these could see the xG go from 0.90 to 0.60, for example. If a player decides to attempt a bicycle kick from a long way outside of the box with their back to goal, then their xG for that would likely be closer to zero than it would be one.
Why Is xG Important?
Expected goals can become an important factor in a number of different ways. For a coach, it can be beneficial for them to utilise the metric when they come to evaluating team and individual performances. From the teams standpoint, it can display the type of chances they are creating. For example, if the xG is low because their chances are seen as closer to impossible than they are certain goals, then the management can look at the factors that have made it this way.
Pinpointing a Team’s Ability
From there, a coach can pinpoint what a team is doing well or poorly. So, if the expected goals are low because the angle is always tight and the chance is never a one-on-one, then they can look at ways to rectify this. On the other side of things, the expected goals metric can be used to measure how a defence is performing. In theory, the lower the xG, the more difficult the defence is making it for the opposition.
This can be a tricky rating to use though as a defence may keep a clean sheet in a game, but it could be because their own attack kept the ball for most of the game, making it impossible for the opposition to go on the offensive. In the same way, the opposition side may just have an awful game going forward, meaning that the low xG is more of a damning indictment of how they played rather than a compliment to the defence.
Working Out Why Teams Have Struggled or Excelled
From a statistical point of view, this can be used to work out why teams have struggled or excelled in a season. If a team has a very low xG, it stands to reason that they will have scored fewer goals and will likely sit further down the table. However, as already mentioned, a high xG does not mean a lot of goals will be scored, while a low xG does not mean fewer goals will be scored. After all, football is a game that has seen its fair share of ‘impossible’ goals and moments.
Delving into Qualities of a Player
This measure can also be important to delve deeper into the qualities of a player, away from the simple goals and assists metrics. If a player is continually given the ball in difficult situations due to poor play from those behind them, then it stands to reason they will score less goals than a player that is continually provided with excellent chances. Therefore, the xG measure would show a coach that the player is being given tough chances, which is why their total goals are lower. They can then utilise this information to try to get that player into more positions that are conducive to a higher chance of scoring goals.
If a player has a low xG but has scored a lot of goals, then this could allude to them being something of a star on the ball, harbouring the ability to turn a situation in which they are unlikely to score into one where they do score a goal. The likes of Lionel Messi and Cristiano Ronaldo spring to mind when you think of a goal being scored from nothing.
Giving Bettors Better Insight
Expected goals can be useful in a betting sense too. This can give bettors more of an insight into how particularly teams are expected to perform in front of goal. If a side has a high xG while they have kept the expected goals of their opponents very low in every game they have played in, this might allude to a handicap market being ideal as they supposedly create quality chances whilst making it very difficult for their opponents to create.
Is There an Expected Assists (xA)?
Yes, the expected assists (xA) metric is one that is used to measure the creative output of a player by looking at how likely a key pass will become an assist. Like expected goals, this measure depends on a number of factors including the placement of the finisher, the pass, accuracy and speed of the pass and more. This model does not usually take into account whether a goal or even a shot was taken. Instead, it simply looks at the quality of the pass itself.
This measure can be very important as if a very creative player is being let down by their forwards, then this can still show their coach and critics that they are creating good chances. For example, if a player has a high xA but low assists, it stands to reason that the players they are trying to assist are missing their chances.
Should I Take Expected Goals as Law?
No. We cannot stress enough that just because a team has an xG of 2.8 in a game, it does not mean they will score three goals. In fact, they could score a lot more or a lot less. The expected goals metric is not something that should be the one and only rating you use. Much like in any sports betting or otherwise, it is incredibly important to ensure you are using other metrics to ensure you get the full picture.
Use Only As Supplement Information
In a betting sense, if you wanted to bet on Manchester City’s goals against Chelsea, you will need to take a number of factors into account. You can of course start with the xG of both sides, but you should not stop there. Next, you would move onto how many goals the sides have scored and conceded in the season, their scoring form in recent weeks, historic results between the sides, the state of play in the game, whether top scorers, midfielders and defenders are missing/performing and more.
Ultimately, if you are going to base your betting or pub talk solely on xG, you are going to get stuck at some point. Instead, xG is a metric that should be used to supplement knowledge of the game.
