[Football] Who actually produces and publishes xG data?

[zefarelly]

In some ways it is / is going to make the game more streamlined so I agree to some extent. But still pretty far from reaching that stage, teams still play with different styles and unless you get into it, its pretty avoidable to get bothered by it, unlike VAR.

spontinaity and the element of suprise is often what make sport so exciting. take it away and we may as well watch painter racing with the gloss touch dry challenge!

 

[Richy_Seagull]

xG also has one very major flaw. It's a snapshot that takes no account of the overall situation. Example - Burnley scored against Southampton last year, direct from a corner. Westwood took it, it bent in at the near post, the keeper flapped it clear, Ben Mee banged it in from less than a yard. xG gave it 0.01 because the ball crossed the line before the keeper knocked it back, and direct from a corner has very low xG. If the keeper had stopped it crossing the line or goalline technology hadn't worked, then Westwood's 0.01 xG wouldn't have counted but Mee would have had about 0.9 for his. It's pure nonsense to say that if the goalkeeper had made a save, Burnley's expected goals would have been higher than if he had let it in.

Same from a free kick. The forward has a shot with xG 0.1 and it goes in. The attacking side has no other attacks and their xG for the game is 0.1. But if the keeper makes a brilliant save and tips it onto the post, only for it to rebound to the middle of goal and the onrushing forward taps it in from half an inch, the xG goes up to 1.09 (0.1 + 0.99). That keeper's save has cost his side an expected goal; if he had let it in, his side might have conceded a goal less!

Couple of things. xG has a built in model that prevents any way a single chance can have an xG of over 1 in one period of play like you describe. Also, of course its not perfect, and there are situations where it doesn't completely reflect what happened. But over the long term it is actually very accurate and by far and away one of the most accurate systems to measure long term performance.

 

[BN9 BHA]

When I see xG mentioned.

 

[Pevenseagull]

I get the concept of xG but I don't understand who actually publishes this data. Is it one source? It is multiple sources?

Is it a 14 year old kid with a laptop in Manilla who's never seen a PL game in his life, watching matches on a dodgy stream and feeding his view of how easy / difficult a chance is into a database, based on playing FIFA? Or is xG the output of a panel of former pros, forming a balanced judgment based on real life experience of playing at the top level? Or something in between?

I think xG has a role but who actually curates the data and forms the judgements? It seems to becoming accepted as a universal truth and valid measurement to judge teams and players and the numbers go unquestioned; but I've no idea who is actually responsible for producing, publishing and managing the data.

 

[Audax]

I think the problem is that xG purports to represent each side's chance of victory, and it doesn't. In a hypothetical match played in the centre circle where each side has only one attack, a free kick with an xG of 0.1 Both sides score from the free kick. And yet the xG doesn't say a 0.1-0.1 so a draw is a fair result; the xG says Burnley (the team with the better goalkeeper ) had an xG of 0.1 while Brighton (whose goalkeeper didn't make the fabulous save) had an xG of 1.09. Is it really fair to say that Pope's hypothetical save made a big improvement to Brighton's chances of winning?

And here's a perfect example of why statistics often get a bad name. Because the average Joe on the street makes assumptions about what the statistic is actually for and then goes off on a wild goose chase.

xG *does not* purport anything like what you say. Some *people* interpret the metric in that way. And, IMO, they are wrong to do so. Like any metric / statistic, xG can be used rightly or wrongly. xG in and of itself, however, is just a set of numbers produced by statistical analysis that attempt to rate the "quality" of a shooting chance. It purports nothing more or less than that. It doesn't even take into account the skills of the individual players involved in any specific shot - it's purely about how likely a shot taken from a given position, with a given angle, and etc (the data points they use in the analysis) is to end up with a goal.

You can then use that metric as a basis for further analysis. So if a shooting chance is scored as an xG of 0.5, all that tells us is that *historically* similar shooting opportunities have resulted in a goal 50% of the time. Sometimes that shot will have been taken by a midfielder with a history of snatching at shots and sending them too high/wide, sometimes that shot will have been taken by a seasoned, skillful, striker with the composure to stroke it into the top corner. But if you wanted to know how good a specific player is at putting away goals when given the opportunity, you can look at all that players' shooting chances and what the xG was of those chances. You can then tally up how many he actually scored, and how many he didn't. You can do that for other players as well, and suddenly you're looking at a metric that becomes useful in comparing the quality of players.

It's still useless as a tool to predict a side's chances of victory. But that's because it's a metric that always looks backwards, not forwards.

 

[Dorset Seagull]

Are we not in danger of becoming more concerned with computer based predictions than with the actual happenings on green grass by human blokes ?

Blame Billy Beane

 

[Giraffe]

You can always come up with examples of where XG doesn't make sense but as an overall pattern over several matches or a season it will give a very good guide to how a team creates and converts chances and defends.