As with everything, Expected Goals comes with a number of limitations.
Expected versus actual
While Expected Goals are a good predictor of a result, actual goals do not always align to the expected goals. The Champions League final being a prime example. However, over the long run, the number of goals a team scores tends to revert to the Expected Goals. If a team is scoring a lot more than the Expected Goals, there is a good chance they are overachieving. Therefore they could be a good option to lay in future games, especially if they come up against the opposite situation – a team who is creating lots of good chances but not putting them away. For example, laying Man U towards the end of their good run of form (after OGS was appointed).
Player skill levels
The other main limitation is Expected Goals does not take account of the player taking a shot. As any football fan will know, all players are not created equal. For example, Vincent Kompany’s goal against Leicester which turned out to be crucial in the 2019 EPL title race only had a 1% chance of going in. As this % chance is an average of all types of players, the true odds were probably a lot lower. If Kevin De Bruyne was taking the shot it would still be classed as 1% but the true chance would probably be closer to 5%. Again, over the long run Expected Goals should revert to actual goals but there can be short term fluctuations.
Watch the match
Trying to predict the future purely based off history is not going to provide perfect results every time. While xG provides a good indication of which team should be on top, I advise people to watch the match before putting on a trade. This provides crucial insights into whether the team we expect to win are actually playing like their form suggests.
The below sections provides some further analysis and answers a number of common questions.