Wolf1877 wrote: ↑Wed Jan 03, 2018 11:55 amThere are some glaring failings in xG in my view. So I'm sort of half in Stelling's camp.
For a start off there is the accuracy of the xG model's measure of the goal attempt probability. I think Peter pointed out the extreme long range Wayne Rooney goal for Everton v West Ham failed to take account of the fact that the keeper was 30 yards away from his goal. The Mahrez goal for Leicester v Spurs in November from just outside the penalty area was assigned a 0.04 probability by understat (i.e. odds of 25) which in my opinion vastly understated the probability of a player of Mahrez's quality scoring from that position with the Spurs defenders all standing a couple of yards off him. I'd estimate the probability in that case as somewhere between 0.1 and 0.2 rather than the 0.04 calculated by the understat algorithm.
I think, this (the Mahrez bit) is common misunderstanding. One of the reasons xG came to was that someone realized that there's very little difference between the outcome based on who takes the shot. So Mahrez's ability has little impact; at least so little that it's statistically irrelevant. Sure there'd be a difference between Gary down from the pub and Mahrez. But Mahrez, Kane, Aguero, Shane Long, Rondon, or Vokes matters little. The difference is the better players play in better teams, that move the ball faster and gets it into more promising positions, whereas say Rondon will just whack it at goal at first sight. Look at someone like Messi. He very rarely takes on difficult shots, for the simple reason that he doesn't have to. Kane use to shot at everyone that moved a few years ago, but has also become much smarter with his finishing as the team has gotten better.
As for the Rooney thing, that's such a rare instance that it matters little.
Not really a flaw in the system imho and it evens out over a full season in most cases. Say there was a 1 in 10 chance that Soton took an early lead, it should be factored in for future calcs, as there'll be a 1 in 10 of something similar happening in future matches of about the same character.Wolf1877 wrote: ↑Wed Jan 03, 2018 11:55 amAnother factor that xG fails to recognise that teams tend to up the tempo and attack when they need a goal and tend to defend when they dont need a goal (unless they are completely dominant like Man City). So the timing of goals will greatly affect the xG stats for most teams IMO. So taking last nights Southampton v Palace game. Southampton went ahead on 19 minutes. After that point Southampton's xG barely rose for the rest of the game. I suspect that Southamptons xG would have finished far higher if they had not scored so early as on 19 minutes as they would have likely carried on attacking Palace and accumulating a bigger xG for longer until they actully scored.
I don't know where you're "a dozen matches" from? The standard to calculate it is by looking at the thousands upon thousands of matches since Opta starting recording shot time, shot position etc. and then finding out how big a percentage of shots goes in, when the position, condition, shot type, pass type etc. are the same.ShaunWhite wrote: ↑Wed Jan 03, 2018 3:56 pmIs there even a standard was to calculate xG ?
Who's decided that when Sterling passed the ball last week he'd created a 30.2764% chance of a goal being the outcome ? To even suggest you can calcuate that from even a dozen matches is slightly sillly gven the huge number of variables, pitch condition, opponent strength, his current 'form' etc etc etc. I know these things aren't about 1 game, it's about averages, but sample size and variance have been conveniently ignored.
Incidently, given that you're a trader in the NBA, you might be interested to know that NBA teams are to varying degrees using a Basket version of xG. My understanding is that a lot of teams have toned it down from being the whole foundation of their set-up, but it's still significant. Most prominently in how the numbers of 3-pointers have risen dramatically. A team like the Cavs will take 3's all day long with noone to rebound, if it's the percentage-wise best shot. And someone like James Harden has spots, where he'll never ever take a shot from because the xG is poor.
Reverse engineered is simply wrong and, frankly, a gross misunderstanding of how it's compiled and works. I fail to see how it would at all be uncomprehensable that most teams weren't likely to score more than 3.5 goals (as calculated after the fact). I've watched a ton of football in my time, and most matches were a team gets 4+ goals, they were getting the rub of the green with at least some of their goals, and scoring a higher percentage than what I would from a purely logical standpoint (as opposed to after the fact statistical). It's a huge rarity that I'll watch a match where one team gets 4+ and I'm left thinking, they could or should have had more than that based on the match.ShaunWhite wrote: ↑Wed Jan 03, 2018 3:56 pmxG also seems to be designed to produce inherently plausible results. A scale has been invented to say a team should have scored between say 0.5 and 3.5 goals (i don't see many xGs much higher) in a sport where teams usually score between 0 and 4 goals. It's a figure that's been reverse engineered. It's always going to be quite close but give the impression that there's something to be learned from the modest variations.
A quick check of number of goals by each team in PL this season for 0, 1, 2, 3, 4+ respectively is 32, 30, 21, 9 and 8 % with similar numbers last season, so lumbing 0-4 goals together as what teams score is a bit simplified imo.