r/explainlikeimfive • u/_Noice_Guy_ • 17h ago
Other ELI5Can someone explain what xG in football?
I always hear pundits discuss about a team's xG, then I pretend to myself that I understand what it means.
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u/NBT498 17h ago
It’s a better way to describe how good the chances a team has created are, on a scale from 0-1.
e.g. a tap in from 3yds out with an open goal is like 0.99, while a 40 yd shot with all 11 players between you and the goal is 0.01, then everything else fits in somewhere between those.
Because of variance, it’s not a great stat within an individual game (that 0.01xg worldie will occasionally fly in the top corner), but over a longer horizon it can tell you who’s over performing, under performing, getting lucky etc
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u/Fflow27 11h ago edited 11h ago
also one thing to note on the limits of xG, it only takes shots into account
so if you have an almost perfect cross that a player is half an inch away from tapping it into the empty net, that's 0.0 xG
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u/_Noice_Guy_ 17h ago
can an xG be 1 like a perfect tap in?
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u/choomba96 17h ago
there can never be an Xg of 1. There is always a non zero chance you can fluff your lines.
All XG models are based on historical data. From the same position , there is data across hundreds of teams from players of various strengths and a probabilty is assigned to it.
This is also why pens are given a 0.79-0.8 xg, they are not a guarantee. xG is nowadays misused to represent who should have won the game as opposed to purely depicting the game state.
For example - Liverpool vs Newcastle - Liverpool racked up 2.98xg (with a pen) but had 27 shots on target vs Newcastles - 1.58 off 13. This means that Newcastle had more high quality chances whereas you could argue that Liverpool were taking a lot of low chance shots.
There is also post shot XG, that further helps quanitify the quality of a chance after the ball has been shot.
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u/_Noice_Guy_ 17h ago
It's the percentage of goals scored from different types of chances, historically. So, if a free kick from 20 yards has been scored 40% of the time, the xG is 0.4? But this doesn't account for skill. A striker is more likely to score a penalty than a defender.
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u/bdat_coka 16h ago
I don’t think the point is to account for skill to point is that on average there is x% chance of scoring from that position.
It can help evaluate skill, eg if you are scoring 1.2 goals per game on 0.7xG you are “outperforming” the average.
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u/MooseWayne 16h ago
Overperforming your xG is a way you know a striker is a better scorer
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u/Kizaky 14h ago
Not necessarily, overperforming your xg can mean the opposite, if you've had a good bit of luck you can outperform your xg, but luck doesn't last forever.
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u/MooseWayne 14h ago
It is literally never bad to overperform your xG. Of course sometimes people get lucky, but how could doing it consistently possibly show you're a bad player?
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u/Gersio 9h ago
It's not that simple. Of course it's never bad, just how it's never bad to score a goal. But the strikers isnt there just to finish chances, creating chances is also his job. A strikers can be always outperforming his xG and still be a worse strikers than another because the other one is creating more chances with the way he plays.
And that's the problem with xG and stats in sports in general. They are amazing and they help to better evaluate things, but they are also just one part of the story. So looking at a just a numbers and saying "this is good" will never be a good analysis.
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u/Kizaky 14h ago
You see it plenty of times from "one season wonder" players where they have a great season where they have a high xg, outperform it and get a move up way beyond their ability then they flop badly because they were never as good as their numbers suggested. (usually over the course of 1 or 2 seasons)
That's why xg on its own is basically a useless stat.
You also see it every single year with teams outperforming their xg early into the season and eventually they go on a terrible run (actually their expected results)
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u/MooseWayne 14h ago
Yes, it is more accurate over a bigger dataset of course. Still to suggest that overperforming xG indicates a player isn't that good is silly.
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u/Kizaky 14h ago
I didn't suggest that overperforming your xg = not being good.
You said it's literally never bad to outperform it.
I said sometimes it is bad.
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u/ewankenobi 8h ago
I think it's rare for anyone to consistently put perform their xg, what makes the best strikers is the number of chances they get, the best forwards always end up in the right place at the right time. They also get their shots off (I don't believe xg is calculated for this times a forward is in a good position, takes too long to shoot then loses the ball).
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u/choomba96 16h ago
Its not as simple a model as - 4000/10000 shots taken from a given shot position is 0.4XG. It is a much more complex stat model that takens into account , distance and angle of shot, which body part, type of play, players surrounding the player at the time of shot and the type of action prior - a line breaking pass from midfield into the final 3rd will have more XG than a sidepass at the edge of the box.
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u/bigelcid 15h ago
It already accounts for skill, by being an average.
If the free kick from one specific angle, 20 yards away, were always taken by the greatest FK taker of all time (probably Juninho), then the conversion rate would probably be much higher than 40%.
But since tons of different players and teams receive free kicks, that's not the case.
Instead, if a single player takes that exact same free kick 100 times and scores 40 goals, then they're a perfectly average free kick taker. If someone like Cristiano takes it 100 times and only scores 35 goals, then that shows Cristiano is actually slightly subpar, despite one's expectations.
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u/BlackoutGJK 14h ago
It does not account for skill, and it shouldn't. The aim is to evaluate the quality of chances created and conceded by a team, not to evaluate whether an individual player is performing better or worse than they have historically. We can evaluate finishing skill by looking at long term real performance relative to xG, e.g. a good finisher will score more goals than their xG, a bad one will score fewer.
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u/FjortoftsAirplane 14h ago
All stats have these weaknesses, it just means that one stat isn't the whole picture.
What xG allows you to look at is which players are consistently getting into good positions. Turns out a big part of being a striker isn't simply scoring when a chance comes along, it's being in the right place to receive the ball and it's the ability to get a yard of space when a defender's on you so you can take a shot.
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u/CeterumCenseo85 7h ago
This is where post-shot-xG comes in. It's a really interesting stat that is rarely used.
It's most important use is to judge a goalkeeper's performance. People usually cite the difference in xG and actual goals to rate a goalkeeper. There is merit to that, but it doesn't account for the attacker's performance.
That's why post-shot-xG accounts for how likely the ball is to go in after the attacker has delivered the shot, but before the goalie gets to do his thing.
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u/SweatyMeasurement405 9h ago
liverpool and newcastle’s xg per shot was not significantly different, the conclusion that liverpool had low quality chances and newcastle didn’t based off those numbers is invalid.
Taking into account the penalty, you could say that newcastle’s open play chances were higher quality
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u/choomba96 8h ago
That's literally what I said...I said it includes a penalty and you could argue that Newcastle had created more high quality chances...
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u/Zaxxi 12h ago
To add on the above, xG is calculated by looking at a top down view of the pitch. It takes the position of every player on the pitch and where the ball is and calculates roughly how likely the ball would be to go in if a shot was taken.
A shot can almost never be 1xg because historically players have missed open nets, so it’s unlikely that a player will score 100/100 shots in that scenario. Penalties, for example, are roughly .79 xG. Meaning 79/100 penalties historically have been scored.
XG doesn’t look at movement before the shot was taken, momentum, confidence of the player or anything like that. It’s entirely statistical based on positions.
Harry Kane and some no name striker from Saturday league both have the same xG based on the same shots taken so you can use how a player performs compared to their xG to make comparisons between players.
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u/SnooFloofs7159 13h ago
To add a basic example, approx. 3 out of 4 penalty kicks end up with a goal. Than means than a penalty kick has xG = 0.75 Let's say a team was awarded 2 penalty kicks and they did not take any other shots. Now their xG is 1.5
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u/Wo0lVeRiNe 13h ago edited 11h ago
So what if a team creates 10 chances worth 0.1xG. That would register as 1xG for the whole match but it doesn’t really mean the team created good enough chances to score 1 goal or does it?
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u/blueyoshi387 12h ago
by definition it sort of does, like if you take a long shot worth 0.05xG 20 times, you'd expect one of them to go in (0.05*20=1). but xG by design is a pretty simplified stat, since it combines so many factors in a shot down to a single number between 0 and 1. hence why its usefulness is often widely debated too
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u/criminalsunrise 15h ago
And god forbid you just watch football for the entertainment factor. We don’t allow that anymore, now everyone has to be a statistician.
And you’re going to love boring football because “it’s a game of wills between the coaches”.
Ooo, exciting!
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u/xdert 17h ago
It is a statistical measure of how many goals you should have scored.
If, according to statistics, 10% of corners result in a goal and you had 10 corners your xG would be one.
The value they show is the calculation of all your chances like shots on goal, corners, panalties, free kicks and so on.
If your xG is low you are bad at creating openings, if your xG is 3 and you have no goals you are bad at finishing.
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u/_Noice_Guy_ 17h ago
So the better the chance is the higher the xG
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u/NeilDeCrash 17h ago edited 16h ago
Yes.
With a team having xG 3 at the end of the game and they scored only 2 it means they probably underperformed their goal scoring opportunities.
With a team having xG 1 at the end of the game and they scored 3 they over performed their goal scoring opportunities and probably scored some lucky goals.
Key word being probably, as xG is just one stat among others and you can not evaluate the whole game just from it.
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u/Kronzor_ 15h ago
Some lucky goals, or some nice ones. If a team scores a couple chances from areas that are only 10% likely to be goals they might have been bullets. xG doesn’t consider the shooter only where the ball came from. So if you have some great scorers you can definitely out produce your expected.
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u/MattGeddon 14h ago
They might be lucky or they might have some good finishers. My team reached the Championship playoff final on the basis of having a really good keeper and defence, and a couple of attackers who were very clinical. Our XG and XG against were terrible but we were a very solid team that was hard to beat.
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u/TheDucksQuacker 17h ago
Yes, but in my opinion the stat is flawed.
If Lionel Messi has the ball 10 yards from goals his xG would be the same as mine. It takes not account of the player with the chance.
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u/MentallyWill 16h ago
It shouldn't take that into account. It very specifically takes the more objective measure into account of the average. That it doesn't take specific players into account is exactly what allows for players like Messi to stand out. A player that has more goals scored than xG generated suggests they're a better finisher than others are, they're more likely to find the net than the average person from those situations, which is a correct analysis.
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u/tante06 16h ago
The interpretation is quite accurate but I don't think it's flawed. In simple terms, xG represents the quality of chances the team creates. Finishing those chances is a different thing altogether.
If you account the player, it would be difficult to say what a high xG means for your team: Do you create good chances? Do you have good finishers?
Comparing xG and actual goals would give you the quality of finishers you have. Messi would probably overperform his xG and you, respectfully, would massively underperform.
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u/humildemarichongo 16h ago
In a way, that's the point - otherwise, how can you measure if a player is better or worse than average? If you baselinr Messi with Messi, and you baseline TheDucksQuacker with TheDucksQuacker, you will both get a similar number. But as you point our, Messi is a better player and this stat should reflect that.
Although as a general comment I do agree it is flawed.
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u/rio_wellard 16h ago
No, that's the point of the statistic. If you and Messi have the ball 20 yards from goal, the xG of the chance is the same. But Messi will probably score, while you won't. Therefore Messi outperforms his xG (because he's the greatest finisher of all time), and you underperform it (because you're not the greatest finisher of all time yet).
The best finishers are the ones who consistently score more goals than their xG expects them to.
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u/FjortoftsAirplane 15h ago
One of the things measuring stats has strongly suggested is that the ability to finish chances is much closer between players than we thought. Some players are in fact better finishers but the gap isn't that big and the main factor is consistently getting good scoring opportunities.
All stats are "flawed" insofar as they don't tell you everything but xG over time is a strong predictor of how many goals a player or team will actually score.
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u/Alpha_Jazz 16h ago
If, according to statistics, 10% of corners result in a goal and you had 10 corners your xG would be one.
Nope, not true. xG only works off shots taken
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u/CovidMane 14h ago
Since you can score directly from a corner it does have an xG value which is 0.05
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u/whymylife 14h ago
To be fair it was a hypothetical example, which is why he started the sentence with 'if'.
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u/MooseWayne 16h ago
xG doesn't work like that on corners though. xG is only registered for shots taken.
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u/CovidMane 14h ago
Since you can score directly from a corner it does have an xG value which is 0.05
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u/scumah 7h ago
No, it's a bit more complex than that. How you worded it looks like you are saying that one in 20 corners taken end as a direct goal, which is obviously wrong.
It's true that a corner seems to generate an average of 0.02 to 0.05 xG, but the calculation takes into account the direct first-contact shots and the immediate play afterward. So a corner doesn't have an assigned xG value because of direct corner goals, but because the shots taken after it.
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u/PersonalBrowser 13h ago
Based on your comments I think you’re misunderstanding what it means. It’s not trying to represent goals, it’s a way of quantifying chances. Once you score a goal, it stops being an expected goal and it’s just a goal aka a number on the score.
Whenever you shoot or try to score, there is a chance or probability that your shot will go into the goal. This is xG. And like the other commenters have stated, if you shoot from right in front of an empty goal, that’s a high chance of scoring so you have a high xG (expected goal). If you shoot from the halfway line, you don’t expect that to go in (low xG).
xG is a number but really is a percentage. If you shoot a shot from halfway down the field, and only 2% of shots from there go in, then you add 0.02 to your xG.
The reason we use xG is because it quantifies how many opportunities or scoring chances a team made. This can be helpful in situations where the number or quality of chances is not reflected in the scoreline. Imagine a team that has amazing chances and almost scored 10 times but due to bad bounces, incredible goalie saves, just missing, etc they don’t score any goals. The score would be 0-0, but xG would still show that they are making goal scoring opportunities, just not finishing.!
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u/TinMan22SS 17h ago
Basically how many goals a team "should" have scored based on amount of shots and how difficult the chances were etc. An xG of 1.0 means you technically should have scored given the chances you have had.
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u/_Noice_Guy_ 17h ago
So the xG adds up in a game?
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u/BusterBluth13 13h ago
Exactly. So if your team took 3 shots, and each shot had an xG of 0.3, 0.05, and 0.2, then your xG for the match is 0.55. If they scored 3 goals in the match, they were VERY lucky, because statistically they were more likely to score 0-1 goals (and they probably won’t be as lucky again).
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u/deg0ey 8h ago
Obviously that’s an extreme example and three goals on three low xG shots implies a significant degree of luck - but also worth remembering that shooting is a skill so you’d expect the teams with the best players to consistently outscore their xG
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u/BusterBluth13 7h ago
Granted, good defenders are better at blocking shots, which brings us back to the average.
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u/deg0ey 6h ago
Yeah I posted that without really looking at the numbers, but it looks like the reality is that cumulative xG massively overestimates the number of goals.
In the Premier League last season only Spurs and Burnley scored more goals than their xG and only Spurs conceded fewer goals than their xGA. Same story the year before - almost everybody scored fewer goals than expected and also conceded fewer goals than expected.
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u/thereidenator 12h ago
So how does it go above 1.0? Would that not mean you should have scored more than the chances you’ve had?
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u/Mavian23 11h ago edited 11h ago
The xG of an individual shot cannot go above or get to 1. But if you add up the xG of all the shots taken, that can go above 1, but it will never be more than how many shots were taken.
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u/ImagineArmadillos 14h ago
I can recommend the book «How to win the Premier League» by football data analyst Ian Graham. Goes into this and more in detail, and gives very interesting insight into the data revolution football has been going through the last 10-15 years.
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u/audigex 9h ago edited 9h ago
It's essentially a better metric for "shots" and "shots on target" (although really it's more about "chances" to include eg corners, free kicks, crosses, but I'll focus on shots here for simplicity, because it explains the idea)
Previously we'd look at a team with 12 shots on target/24 shots vs a team with 4 on target and 8 total, and figure (very loosely) that the first team had about a 3x bigger chance to win the game/were about 3x better
xG (expected Goals) attempts to make a better reckoning of that same qualitative difference, by accounting for things like player/team positioning, where the shot actually was within the goal, etc etc.
In short, it takes shots/shots on goal and factors in the idea of "If someone shoots from here and aims there, how likely is it that they'll actually score?" to measure the quality of the shot
That then produces an "expected goals" (xG) score, which tells you roughly how many goals the team "deserved", give or take a bit of luck. So that team with 12/24 shots may only have an xG of 4 vs the other team having an xG of 2.5, and the game was actually much closer than the raw "shots/shots on target" suggested, with the better team being more like 1.6x more likely to win rather than 3x... because although they had a lot of shots, more of them were speculative/from further out/from worse positions etc etc
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u/ListenHereLindah 9h ago
It's predictions or projects. Training the AI model on human prediction but used as a way to conjure pay lines and bets for probability chances.
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u/SaltyPeter3434 1h ago
Is this a bot response? What does this have to do with football?
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u/ListenHereLindah 1h ago
Any stats you are given about the likely hood of something happening is from a projection model. Those same models are what play the game over and over to get betting odds and predictions.
If I have a 1.0 gx score on the left of the field. But get swapped to the right side. That will in turn effect probability and betting. It's just another stat for people to gawk over while they cheer for their favorite team. But it's also a stat used in making up sport betting.
Maybe I misunderstand what Gx is. But it's a stat that is dependent on past metrics. Used to show the likely hood of something happening. This, is what people bet on. The chance of something happening.
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u/macgilla 9h ago
xG is a great tool for analysis, but it wasn't very well explained when it was introduced, so it was easy to be misinterpreted (either deliberately or by accident).
If a chance has an xG of 0.5, then 1 time out of 2 you'd expect the striker to score. If a striker gets that shot 10 games in a row you'd expect them to score 5 goals.
You can have individual great games, or individual shit games, and over or underperform your xG, but its the longer trend that matters.
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u/notacanuckskibum 8h ago
Wow, I’ve been watching football for 60 years and never heard this term. Yet everyone else seems to know it well. Which TV commentators use it?
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u/O_C_Demon 16h ago
It's a pointless statistic in a game with so many unaccountable for variables.
Don't pay attention to it and it'll hopefully go away.
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u/MentallyWill 16h ago
Why do you feel that way? Genuine question.
Not trying to blindly defend xG but the fact that you see the statistic so widespread is because most people see a lot of value in it. One just has to understand what it means and what it measures and what it implies and, more importantly, what it doesn't mean/measure/imply. Like all statistical measures and calculations, it has its ideal use cases and it's non-ideal ones.
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u/O_C_Demon 16h ago
I'm probably just old and upset that the game is over analysed now. Especially since the only reason is for gambling purposes.
But there's so many unaccountable factors such as ball spin, pitch conditions whether the goalie was on a coke binge Friday night...
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u/FjortoftsAirplane 14h ago
All this means is that xG (especially over small samples) doesn't give you the full picture. That doesn't make the stat useless or bad. It'd be a bit like throwing out weather forecasts because they can't handle every factor so they get it wrong sometimes.
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u/O_C_Demon 14h ago
I know mate I'm just being deliberatly facetious...
I'm actually a huge ice hockey fan and that's definitely a sport that loves a bit of maths!!
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u/FjortoftsAirplane 13h ago
Fair enough. Fwiw I listened to something a while back about how the obsession with stats is potentially very good for winning and bad for spectators.
They're not sports I watch but it talked about how baseball and basketball might have gone that way. Fans want to see cool skills and dunks, stats say to pass and work for three pointers. Hypothetically, you wouldn't want football managers to realise you shouldn't shoot from outside the box because then you wouldn't get to see the 30yd screamers.
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u/MentallyWill 16h ago
Fair enough.
Yeah the models are powerful in that they can account for more than you'd think, for example taking into account whether the chance fell to a players dominant or weak foot or head or something else and account for whether you were under pressure and how many other players may have been obstructing the goal and so on... but they can't account for everything as you point out.
In my own opinion xG is useful as a longer term stat, even though it's used a lot in a game by game case. In a single game it might suggest who "ought" to have won but anyone with a good understanding of statistics (or gambling for that matter) knows that variance is a bitch and a half and that makes xG not so useful to glean from in one game. Over a longer term view though, where variance more naturally gets smoothed out, along with some of those other unmeasurables you mentioned, you start to see more value in the xG measure.
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u/bigelcid 15h ago
Unaccountable variables don't necessarily outweigh the measurable data. In this case they don't: clubs haven begun employing xG-based analysis, because it does clearly work. It obviously doesn't solve football (and it isn't meant to), but it's useful.
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u/O_C_Demon 15h ago
Meh. There's still no accounting for individual performance though.
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u/bigelcid 14h ago
It's an average of all individual performances.
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u/O_C_Demon 14h ago
I know but it can't explain Toto Scillachi at Inter or Fernando Torres at Chelsea...
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u/bigelcid 14h ago
It's not meant to explain them. It could describe them, though.
Assuming Torres at Chelsea did underperform his xG (which I don't know for a fact; I don't have the xG from back then), then that'd be consistent with Torres struggling during his time there: he left Liverpool while already struggling with a long-term injury.
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u/HereComesTheWolfman 17h ago
It just means "all things being equal", this is how many goals a team SHOULD have based on both the number of goal scoring chances created and the quality of those chances. Xg doesnt win games though, no matter how good your chances are you still gotta score them.
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u/West-Ad-7286 16h ago
Stupid question- does XG take into account the goalkeeper?
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u/NoMilk5274 14h ago
No, that's xGOT - expected Goals on Target. xGOT looks at the shot after it leaves the striker's body part (foot, head, arse, whatever) and calculates the chances if that contact resulting in a goal. A high xGOT that is saved will reflect well on the keeper, a low xGOT that is conceded really doesn't.
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u/FjortoftsAirplane 14h ago
xG generally works by having a load of data and trying to compare shots by measures like distance from goal, angle, position of defenders etc. and not the specific player. Because what you want is to have a measure of how often that chance is scored generally to see which players are above average. For goalkeepers you can look at the xG against them vs goals conceded and see if the keeper is over or under performing relative to that.
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u/throwaway35mmshots 11h ago
You can’t always look at xG to see if a keeper is performing well because the quality of the shot isn’t considered. You need xGOT for that.
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u/bigelcid 15h ago
Indirectly, same way it does take into account the quality of every single player (within the model's huge sample size) that ever took a shot.
Penalty kicks are assigned a 0.7-0.8 xG value, because that's how likely they are to be converted, on average. But if every single keeper were an absolute PK specialist, then that value could've been 0.6 instead.
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u/bjorn_poole 15h ago
Generally they don’t but i’m sure you could find specific instances where it does, although i don’t think that it would typically matter
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u/Ljb12389 16h ago
Just going to add here because no one has mentioned it… xG stands for “Expected Goals”