XG?

[glastored]

Please excuse my ignorance, but what is ‘XG’? I keep seeing it mentioned in football analysis, but I’m sure I’m not the only person who hasn’t a clue what it signifies. Can someone enlighten me?

[Super]

1 minute ago, glastored said:

Please excuse my ignorance, but what is ‘XG’? I keep seeing it mentioned in football analysis, but I’m sure I’m not the only person who hasn’t a clue what it signifies. Can someone enlighten me?

The most pointless stat in football expected goals

Edited August 27, 2023 by Super

[Unan]

A useful stat in football that uses more than one isolated variable (such as goals/assists & the Scott comparison) to give a better (far from perfect) understanding of how a teams performing.

Edited August 27, 2023 by Marcus Aurelius

[Ziderarmy]

Can someone explain accurately how it’s calculated as I saw a game last week is was 1-0 at HT and the tens leading had a XG of 0.58

[!james]

https://statsbomb.com/soccer-metrics/expected-goals-xg-explained/

[Unan]

3 minutes ago, Ziderarmy said:

Can someone explain accurately how it’s calculated as I saw a game last week is was 1-0 at HT and the tens leading had a XG of 0.58

Put simply, Expected Goals (xG) is a metric designed to measure the probability of a shot resulting in a goal.

An xG model uses historical information from thousands of shots with similar characteristics to estimate the likelihood of a goal on a scale between 0 and 1.

For example, a shot with an xG value of 0.2 is one that we would generally expect to be converted twice in every 10 attempts.

 

[petehinton]

1 minute ago, Ziderarmy said:

Can someone explain accurately how it’s calculated as I saw a game last week is was 1-0 at HT and the tens leading had a XG of 0.58

Is based on the chances you create, basically. So if you have a penalty, that’ll create an xG of 1 straight away as you’re expected to score in that situation. 
 

Or if a team gets battered, but scores a 35 yard goal, their xG will be tiny as they’ve scored a worldie which wasn’t expected. 
 

People call it pointless, but it’s very very indicative of overall performance and dominance of a game. Give me consistently positive xG over possession any day of the week, as is being shown with a lot of clubs atm as largely useless. 

[bearded_red]

2 minutes ago, petehinton said:

Is based on the chances you create, basically. So if you have a penalty, that’ll create an xG of 1 straight away as you’re expected to score in that situation.

Penalties are roughly 0.8

[Dredd]

It's just another stat pulled out of someone's ass as an indicator of 'good performance' despite losing. 

[glastored]

Thanks for everyone’s replies, but I’m still not sure what I’m reading! Perhaps I should just leave such deliberations to the statisticians among you, and just accept I don’t have that type of mind!

[Davefevs]

18 minutes ago, glastored said:

Please excuse my ignorance, but what is ‘XG’? I keep seeing it mentioned in football analysis, but I’m sure I’m not the only person who hasn’t a clue what it signifies. Can someone enlighten me?

You know when a commentator says:

• that was a half chance, say a one-on-one, xG will give it a probability score between 0 and 1, based on 100s and 1000s of similar chances.  Everyone thinks a one-on-one should be scored everytime.  The reality is, they aren’t.  Aaron Connolly’s chance, on the angle he was on, etc, etc, was a 1 in 9 chance based on similar chances.  It got an xG score of 0.11.
• Nahki’s tap-in goal, a 7 in 10 chance, 0.70 xG.  How was it not a 1?  You saw the miss against Brum!

[bcfc01]

Not a fan, just another subjective metric to analyse a game.

 

[Davefevs]

Just now, bcfc01 said:

Not a fan, just another subjective metric to analyse a game.

 

It’s not subjective, it’s statistical.  It might be used subjectively

[Mr Popodopolous]

It often can be a decent guide over time, limited use in terms of individuals matches or a run of games.

Edited August 27, 2023 by Mr Popodopolous

[bcfc01]

Just now, Davefevs said:

It’s not subjective, it’s statistical.  It might be used subjectively

No, it is subjective.

Who decides what the chance is on a goal attempt ?

It is entirely subjective from its outset.

[spudski]

Funnily enough we had the number 1 stat for XG in the Championship this past week. Unheard of...

[archie andrews]

some super strength cider at the barleymow.......

[Slacker]

15 minutes ago, Dredd said:

It's just another stat pulled out of someone's ass as an indicator of 'good performance' despite losing. 

See Lee Johnson thread for more examples. 

[Mr Popodopolous]

It's not exhaustive but if a side are underperforming ie playing well but mediocre or worse the xG table may well reflect this. Usually this turns around.

Likewise if a side are performing better results v performance wise, the xG table will be likely to reflect this and results v performance may align over time.

A good example was when we were clearly better points v results in 2016. It caught up!

[Davefevs]

21 minutes ago, bcfc01 said:

No, it is subjective.

Who decides what the chance is on a goal attempt ?

It is entirely subjective from its outset.

It’s not a person say they’re deciding, it’s based on data.  There is sometimes human checking, but no different to a supervisor sample checking your work.

[Ziderarmy]

So the XG can never be higher than 1.0 then? 

Edited August 27, 2023 by Ziderarmy

[Leveller]

6 minutes ago, Ziderarmy said:

So the XG can never be higher than 1.0 then? 

Presumably for a single chance,no. But for match it could be several. 

[Davefevs]

19 minutes ago, Ziderarmy said:

So the XG can never be higher than 1.0 then? 

Here’s a screenshot of Wyscout xg from the other night.  There were obviously more shots, but I’d have to scroll down.

[Erithacus]

So the XG is based on historical data, but not an actual game in progress? Seems to be a rough indicator of the game or its aspects. I appreciate the ways stats have been integrated into modern football but I feel the application is far from comprehensive. In short, stats are no predictor of a given moment.

[transfer reader]

14 minutes ago, Erithacus said:

So the XG is based on historical data, but not an actual game in progress? Seems to be a rough indicator of the game or its aspects. I appreciate the ways stats have been integrated into modern football but I feel the application is far from comprehensive. In short, stats are no predictor of a given moment.

xG is never a predictor of a given moment

It tells you the strength of a given chance in a game based on past events

 

However it can (with other knowledge) be used to make other predictions.

Let's say a side has an xG of 20 over 10 games, but has only scored 6 times

Meanwhile their xGC (goals conceded) is 10 and they've conceded 20.

 

You could reasonably* suggest they've created a lot of good opportunities and been very wasteful in front of goal, while conceding goals more often than they 'should'.

So you could make a prediction that the poor form those goal numbers would lead to is not really a good representation of the level of performance in the game and that their form will soon turn around. (In fact those arbitrary numbers would be decent even for a team comfortably at the top end of the table)

 

*I say reasonably because in theory you could have 200 shots from 30 yards out with an xG of 0.01 each and get an xG of 2 for a game, but I'm sure we can all agree that's unlikely. Meanwhile a team could have 2 shots all game and an xG of about 1.5 if they were both penalties

Like any stat it can be useful, but doesn't mean much in isolation. It's how you use it with other stats that matters

Edited August 27, 2023 by transfer reader

[And Its Smith]

In theory it’s good. In practise it’s flawed.  One example is if there is a goal mouth scramble with 3 shots a team will get an xG for all 3 shots which doesn’t make sense as if the first shot had gone in then the other two wouldn’t have happened.  It would better if it judged chance creation better as well. Newcastles goal today would have an xG but they haven’t created that chance.  Finally, a shot from the edge of the box with 4 defenders between the shooter and the goal has the same xG as if there are no defenders there.  Hopefully xG will be improved one day as the principle is a good one 

[Mr Popodopolous]

Here is a worked example.. I'm using more basic shot data.

Norwich City. 4-4 draw, 4-0 win. I won't bother to filter out shots on target v penalties. This is their two away games.

24 Shots for, 49 conceded.

Shots on Target- 11 for, 16 against.

Logic dictates that they are outperforming on the road. Vastly. Interesting to see if they will keep it up..not so much the goals but the allowing so many shots and chances on the road.

It does suggest they can be got at, on the road especially. Very clinical at the same time but again that is well above the norm.

All the same, the Sofascore (so probably quite basic) xG..has them winning at Huddersfield but losing the 4-4.

Edited August 27, 2023 by Mr Popodopolous

[Davefevs]

15 minutes ago, And Its Smith said:

In theory it’s good. In practise it’s flawed.  One example is if there is a goal mouth scramble with 3 shots a team will get an xG for all 3 shots which doesn’t make sense as if the first shot had gone in then the other two wouldn’t have happened.  It would better if it judged chance creation better as well. Newcastles goal today would have an xG but they haven’t created that chance.  Finally, a shot from the edge of the box with 4 defenders between the shooter and the goal has the same xG as if there are no defenders there.  Hopefully xG will be improved one day as the principle is a good one 

….and that is why for ages I’ve been trying to say to you it’s not good for judging the result of a match, and why xg tables based on individual games are pretty futile.  But it is useful for individual player scouting for example.

Worth you looking at Statsbomb’s xG it is far more sophisticated than any others and solved many of the probs of previous models, inc defenders and goalkeeper positions.

[bcfc01]

An algrorithm based on historical subjective data.

I love stats and I use them everyday, but I don't like subjective stats.

 

 

[IAmNick]

1 minute ago, bcfc01 said:

An algrorithm based on historical subjective data.

I love stats and I use them everyday, but I don't like subjective stats.

 

 

How is the historical data subjective?