xG’s: Why it is flawed and pointless

5 Replies

Over the last few days I have had a few debates with people as to why the “expected goals” (xG) statistic is pointless. It seems there are plenty who defend it.

To work out a team’s xG for a match, every shot must be analysed and given an “Expected goal value” (EGV).

EGV is the probability that any given shot will end up as a goal.

As Patrick Lucey, director of data science at STATS, explains, EGV is based on a number of factors, such as where the shot was taken from, the proximity of defenders, the nature of the attack (i.e a direct free-kick or a penalty). The EGV of a shot assumes it is being taken by someone of average ability in the league, so it expects for instance that a shot from 10 yards out plum in front of goal with no defenders nearby has a high chance of ending up as a goal.

People then use this statistic to highlight sides who are over performing or under performing.

One journalist from the Mirror recently wrote an article stating that Arsenal should only actually have scored 11 goals this season, not the 19 we have actually scored. And should be mid table.

Some outlets are actually printing a weekly xG table in an attempt to highlight where sides will be based on if shots that are expected to go in did, and vice versa. Arsenal are currently 11th in the league based on expected points.

The glaring issue with xG it is that it assumes every player is born equally.

The xG value of shot assumes it is being taken by someone of average ability in the league.

It puts the same value on a chance going in regardless of if it is Alexandre Lacazette taking a shot or Yaya Sanogo.

And it assumes the same with a goal keeper.

It does not account for David de Gea being a better goal keeper than Manuel Almunia . It assumes that a shot is just as likely to beat both.

So Sanogo against de Gea is given the exact same value as Lacazette against Almunia.

Football is all about exceptional talents. It is what separates a £50m striker or a £80m goal keeper from one that costs £5m. It is why Manchester City top the league and Newcastle are bottom. Why Sergio Aguero outscores Joselu.

Lionel Messi has “outperformed” his xG prediction throughout his career. That is because he isn’t average. He is the greatest player of his generation. He should not be compared to the average within the league.

As soon as you start disregarding individual talent and assume that everyone is the same, your analysis becomes flawed.

So Arsenal are performing above what their xG believes they should have scored.

Perhaps the truth is that in Alexandre Lacazette and Pierre-Emerick Aubameyang; we just have above average strikers?

Ultimately, xG is pointless. It means nothing. It is a spreadsheet which, after a game, “predicts” what should have happened based on the law of averages. Not actually what happened.

And it is what happened that is ultimately important.

A world class goal keeping performance, a keeper letting the ball through his legs, a striker missing a tap in or scoring a worldie. A beach ball entering the field of play. These are all actual events. Events an algorithm can not predict.

If you wish to give weight to xG’s, that is your prerogative. But I am going to live in a world of actual goals. Actual results. A world that rewards the exceptional talents over the average.

xG is pointless.

Keenos

Match Report: Fulham 1 – 5 Arsenal

2 Replies

Fulham (1) 1 Arsenal (1) 5
Premier League

Craven Cottage, London SW6
Sunday, 7th October 2018. Kick-off time:12.00 noon

(4-2-3-1) Leno; Bellerín, Mustafi, Holding, Monreal; Torreira, Xhaka; Mkhitaryan, Iwobi, Welbeck; Lacazette.
Substitutes: Martínez, Sokratis, Ramsey, Guendouzi, Lichtsteiner, Aubameyang, Kolašinac.
Scorers: Lacazette (2), Ramsey, Aubameyang (2)

Referee: Paul Tierney
Attendance: 25.401

Good to visit the newly-promoted Cottagers again after a four year hiatus when they lost their Premiership status at the end of the 2013-14 season after a series of unfortunate results. Of course, older supporters will remember their salad days when Johnny Haynes and 1966 World Cup winner George Cohen graced their ranks; and it is not to be forgotten either by those of us who both know and care when star striker Ronnie Rooke was signed from the West London club at the grand old age of 35. Not only did he help Arsenal keep their First Division status in a difficult season, but more importantly, his 33 goals in 42 matches (which is still a post-war record that holds firm today) propelled us to our sixth championship win in eleven peace-time seasons back in 1948. Indeed, his amazing overall goalscoring tally of 70 goals in 94 matches is surely one that our modern strikers should rightly aspire to.

Despite the absence of Mesut Özil from the team today due to a back spasm, the omens looked good for us in this autumnal Sunday lunchtime by the Thames in leafy SW6, and sure enough everything started off looking as a typical London derby should look to us all. At first The Cottagers looked confident on the ball, but they appeared to have no final product, therefore Bernd Leno had no particular issues in dealing with the shots that came his way in the first quarter of an hour or so. Slowly the tables were turning. Our midfield men were starting to pick up the Fulham stragglers that were hanging around looking for spare balls that never came their way, and within no time at all we left our mark on the match with a superb goal of movement that involved speed of thought and action. Alex Iwobi collected the ball on the left, slotted it to the overlapping Nacho Monreal who wasted no time in getting it across to the waiting Alexandre Lacazette, who held off the Fulham defender, span around and put the ball in the back of the net in the space between the goalkeeper and the near post. We then started to dominate the match; but unfortunately an error in midfield led to Fulham bringing the scores even just before half-time. Within five minutes of the restart, Alexandre Lacazette got his second of the match with an absolute peach of a goal that came about from a botched throw-in from the home team. Lucas Torreira won the ball, placed it high for Danny Welbeck to nod it down into the path of Alexandre Lacazette, who struck it sweetly for 20 yards into the back of the net. And from then on, it was Arsenal all the way home. Aaron Ramsey and Pierre-Emerick Aubameyang were the substitution pairing after the hour, and the game changed yet again; Aaron Ramsey arguably got the goal of the game after a series of clever headers and neat flicks from his team-mates found him just outside the six-yard box and he nonchalantly himself flicked the ball past the goalkeeper into the net. Now the turn of our other star striker, Pierre-Emerick Aubameyang. He collected the ball from a cross by Hector Bellerín, pirouetted and smashed our spherical friend into the net for Arsenal’s fourth of the day. After more sterling work in the midfield, Pierre-Emerick Aubameyang got the final goal of the day, with a low shot into the corner of the net. Fulham 1, Arsenal 5. Nine wins in a row, six in the Premiership. Wow.

At last, the genie is out of the bottle, as the real Arsenal have finally stepped up to the plate with style and class. Utterly ruthless, this is the mark of performance that we need to see from now on. Every man played their part, every man was well drilled and completely professional. So many great performances across the pitch, it is almost impossible to pick out an outstanding player, but it has to be said that Alexandre Lacazette was immense today. He was coming back to assist in midfield, collecting the ball in the channels and running with oppostion defenders taking them out of position for his colleagues to have a chance to shoot at the goal; a wizard, a true star. A message for the Premiership; Arsenal are back. Be scared, be very scared. Remember everyone, keep the faith, get behind the team and the manager, as these early days are going to be crucial for our future success in all competitions. Stick with the winners.

Victoria Concordia Crescit.

Steve

Arsenal and Manchester United set to repeat Alexis swap deal?

3 Replies

After Wednesday’s blog discussing the need for more balance in the team, one name popped up a few times in my mentions who would be potentially ideal.

Anthony Martial

It is easy to forget that the Manchester United winger is still just 22. He is a year youger than Raheem Sterling, and just a month older than Leroy Sane.

The Frenchman is on par with both with regards to natural talent.

He has had a tough time under Jose Mourinho which has clearly affected his confidence. But this is a player who hit 19 goals in his debut season in England. A figure that puts him between Sterling (23) and Sane (14) for most goals in a season

The statistics for the 3 are not too far off each other. Obviously they can be looked into deeper, breaking down per minute, etc, but I really do not have the energy for that.

Sane is the stand out player when it comes to stats, but he also come to the Premier League as a more established player, where as Sterling broke through as a baby at Liverpool and Martial joining Manchester United as a teenager.

Martial  was always going to have an uphill struggle to justify what seemed like a heafty price tag back in 2015, and this still unfairly hangs over him. Yet two summers on from his big money move from Monaco, is the £36m Man U paid for him really that expensive?

A year later City paid £37million for Sane. He might have been a year older than martial at the time of signing, but had pretty much played the same top level games (Sane 47 Bundesliga games & 11 goals; Martial 49 Ligue 1 games & 11 goals).

Martial was merely the first. He set the benchmark of what players considered as the most talented teenagers in football would cost.

Whilst there has been plenty of talk and speculation over the destination of Aaron Ramsey in recent weeks, and months, Anthony Martial’s contract is also set to expire at the end of the season.

Like with Ramsey, it looked like the two parties were close to agreeing a new deal over the summer, but I am unable to find an article confirming it is complete.

Martial and Ramsey swapping clubs would suit all parties. Arsenal need a wide man, Man U need a central midfielder.

Ramsey would suit Manchester United more than Arsenal at this moment in time. Likewise Martial would fit into the Arsenal time like a glove; providing the width and pace we currently miss in wide positions.

Instead of losing a player on a free at the end of the season that they do not really require, both clubs would be gaining a player that they do want.

From Aaron Ramsey’s point of view, Manchester United are probably the only Premier League club who could match his reported £200k wage demands and offer him the first team football he desires.

As for Martial, Arsenal could dramatically increase his current £75,000 salary to the £115k region, keeping him within the current wage structure.

Martial would get the play time his talent deserves and a coach that promotes free flowing attacking football.

It is a deal that the more I write about, the more I speculate, just makes sense.

Keenos