xG - hype or valuable stat?

[duncanjwitham]

1 hour ago, David said:

However I would concede that rebranding it as chances created cC, or even opportunities, I would have less issues with it.

This idea that a player is in this position on the pitch, he’s “expected” to score is as I say nonsense and disrespectful to defenders and keepers.

I think you're misunderstanding what they mean by "expected".  It's not saying "a player in this position is expected to score", it's more like "if a player had this chance 100 times, how many would you expect them to score".  It is factoring in keepers and defenders because their positions etc all feed into calculating what that expectation is. 

Using "expected" in this sense isn't really branding, it's a really basic statistics term that used all over the place, both in sports stats and stats in general.

[richinspain]

I don't know if it's already been talked about, but how exactly do they calculate the xG for a particular shot? It's being stated that it's the probability that a player in x position, with goalkeeper and defenders in various positions, is going to score a goal. How are those positions calculated? Is there someone at the match calculating each opportunity? Is all done via cameras? Is there a super computer that actually does the calculation? Or is it some geek who decides "ooooh, I'll mark that as a 0.75467423577 xG because that striker's really good and the defending team are rubbish"?

[duncanjwitham]

55 minutes ago, richinspain said:

I don't know if it's already been talked about, but how exactly do they calculate the xG for a particular shot? It's being stated that it's the probability that a player in x position, with goalkeeper and defenders in various positions, is going to score a goal. How are those positions calculated? Is there someone at the match calculating each opportunity? Is all done via cameras? Is there a super computer that actually does the calculation? Or is it some geek who decides "ooooh, I'll mark that as a 0.75467423577 xG because that striker's really good and the defending team are rubbish"?

As far as I know, the positional stuff is all automated.  All the player positions are tracked throughout the match anyway, which is where sites like whoscored are getting their data from.  Players wear monitors IIRC, that tracks their position, as well as heart rates etc.  They are then basically comparing each chance (in terms of attacker position, distance and angle from goal, keeper position, defender positions etc) with thousands of similar chances from other games and seeing what proportion of those chances were actually scored.  This is where the discrepancies come in - how do you decide what’s similar enough to factor into your comparison, or which individual elements matter the most (is distance from goal more important than the angle or not etc).  This isn’t being done for each goal individually though, they will come up with a model where you can feed the player positions in and it will churn out an xG at the end.

Individual player abilities aren’t supposed to be factored in - it’s supposed to be how likely is an average striker (for the level we’re talking about) going to score this chance against average defenders and keeper etc.