Expected Goals are a useful metric over the longer term, because any good xG model will see its players regress back to the mean over time. It means that are certain points of a season we can assess a player's goal total, compare it to their xG total, and identify players who are either over or underachieving the model.
EXPECTED GOALS (xG) assesses the quality of a shot based on several variables such as assist type, shot angle and distance from goal, whether it was a headed shot and whether it was defined as a big chance. From there that shot is assigned an xG score and the cumulative total of these scores provides a team's xG for a match.Stats Definition
The purpose of identifying these players is not to suggest that they are poor finishers or are having bad seasons. On the contrary, the fact that these attackers are regularly finding themselves in good positions to score means, over time, they will eventually start hitting the target.
Getting into the position to score goals is one of the hardest skills in football and so having a high xG but low goal total is much less worrying than high minutes, low goals, low xG. That suggests more of an underlying problem.
UNDERPERFORMING XG: TOP FIVE LEAGUES
Player Goals v xG
| # | Player Name | Team | G | xG | xG Diff |
| 1 | Roberto FIRMINO | Liverpool | 8 | 15.5 | +7.5 |
| 2 | David McGoldrick | Sheffield Utd | 0 | 7.2 | +7.2 |
| 3 | Edinson CAVANI | PSG | 4 | 9.3 | +5.3 |
| 4 | Edin DZEKO | Roma | 12 | 17.1 | +5.1 |
| 5 | Gabriel JESUS | Man City | 10 | 14.5 | +4.5 |
| 6 | Alvaro MORATA | Atletico Madrid | 8 | 12.1 | +4.1 |
| 7 | Diogo JOTA | Wolves | 6 | 10.3 | +4.3 |
| 8 | Lorenzo Insigne | Napoli | 5 | 9 | +4.0 |
| 9 | Nicola SANSONE | Bologna | 4 | 8 | +4.0 |
| 10 | Luuk De Jong | Sevilla | 5 | 9 | +4.0 |
Roberto Firmino is probably having his weakest season at Liverpool from a purely finishing perspective. Much of his xG loss is related to six missed chances within the six yard box with a combine total of around 3.5 xG. He struck the post against Norwich and Bournemouth when it would have been easier to score.
We can probably put this anomaly down to sheer bad luck. Talking of bad luck, poor David McGoldrick. Sheffield United fans adore him for his all-round leading the line performances but it's ridiculous he hasn't be able to convert at least one chance this season. It will turn for him eventually if he keeps getting into the right areas. Elsewhere in the Premier League Diogo Jota's numbers should probably be a little better despite his recent upturn in form, so he is worth keeping an eye on.
Elsewhere, Edinson Cavani has returned from a long injury but has been profligate in front of goal as he looks to get back up to speed - there's no doubt that if he continues to get minutes that xG difference will close.
In Italy the numbers are a little unfair on Edin Dzeko - because of the nature of data gathering he has a few extremely high tariff xG chances that are blocked from close range, but on watching the clips they are far from clear-cut opportunities.
For Gabriel Jesus, there is a missed penalty in there which knocks his xG up by 0.75, while there are a few unfortunate finishes from close range that would normally be converted. Jesus' underlying numbers are just fine, however.
In Spain, Atletico Madrid desperately need Alvaro Morata to step up and make up the gap in his xG; teams that score as few goals as Atleti are hindered more than most by chances going astray.
Lorenzo Insigne 
Chris Wood 
David McGoldrick 
Sheffield Utd 
Milan 
Burnley 
Liverpool 
Wolves 
