xOffside: the stat that measures who knows how to stay onside better than anyone

An assistant referee who, according to the Premier League's own official statement, was too busy watching Erling Haaland to notice that Enzo Fernandez was clearly offside. That's how the recent Manchester derby was decided. And honestly, watching the slow motion clip, it's hard to blame the poor assistant: Haaland adjusting every stride to stay level with the line while Josko Gvardiol pushed forward on the other flank, a half step with his left foot, a long stride with his right, always in exactly the right spot. When Gvardiol crossed the ball over Patrick Dorgu, Haaland was, according to the finest technology available to measure it, three centimeters behind the line marked by the tip of Dorgu's toe.

There are forwards with a special gift for staying on the right side of that invisible line, and they almost never get credit for it. The most famous case might be Filippo Inzaghi, whose habit of playing right on the edge of offside led Sir Alex Ferguson to joke that "that lad must have been born offside." Inzaghi took it personally.

Luckily I was born a meter onside, otherwise I wouldn't have scored most of my goals.Filippo Inzaghi

Can you actually measure who's good or bad at staying onside, and how much that skill is worth? It's strange: for an ability that can decide matches, even titles, there has never been an objective way to separate the Enzos and Haalands from everyone else. People talk endlessly about goals and expected goals, but almost nobody asks how many of those chances simply come from knowing how to stay on the right side of the line.

Counting flagged offsides doesn't solve anything either: knowing that Haaland has been caught offside 25 times in the Premier League doesn't tell you whether he's better or worse at this than Crystal Palace's Ismaila Sarr, who has the exact same number. Cross-referencing it with goals scored starts to make more sense, and that's where an interesting fact emerges: Haaland racks up that mountain of goals while being caught offside only slightly less than Nottingham Forest fullback Daniel Munoz. A simple ratio of offsides per goal doesn't work either: Jorginho scored 21 goals between Chelsea and Arsenal without being flagged offside a single time, which sounds impressive until you remember that almost all of those goals were penalties.

How xOffside was born

The sharpest way to measure this is to look closely at every single pass: its speed, its height, its angle. A long, fast pass is far more likely to produce an offside than a short one; a pass that's lofted or has curve on it, more than a ball played along the ground; and a vertical pass toward the middle of the field is more dangerous than a diagonal one played out wide. Factoring in the phase of play helps too: a long pass during the build up phase is almost three times more likely to end in an offside than an identical pass once the opposition has already dropped back.

Then there's the matter of who's defending. A high line and an offside trap aren't the same thing: some teams have center backs who drop off tracking the runner, while others step up together, committing fully to the trap. Unai Emery's Aston Villa are big believers in that trap, just as Pep Guardiola's Manchester City used to be. But that's not the only approach: last season's Arsenal defended with an extremely high line while catching opponents offside 40% less than the model expected, putting them among the teams that rely least on the trap, just below Sunderland.

To turn all of this into a per player rating, one last adjustment is needed: the position of the receiver. A classic trick for beating the defense's offside line is to start the run from deep while a teammate distracts the center backs, which puts strikers at a disadvantage compared to wingers and attacking midfielders when measuring this skill.

And the result? Not even Haaland comes out on top once position is factored in. The best asset in the Premier League at staying onside is, by a narrow margin, Morgan Rogers (formerly of Aston Villa, now at Chelsea), who has been caught offside just twice on open play passes over more than three seasons, compared to the 18.3 offsides the model expected for him. Alongside Rogers and Haaland, there are at least seven other players who clearly master this art. At the other end of the scale, only nine players are clearly above average, and two of the worst cases happened to play for Tottenham, which probably explains quite a bit. Dominic Calvert-Lewin (Leeds United) and Jean-Philippe Mateta (Crystal Palace) narrowly avoid landing among the ten worst, thanks to Yoane Wissa (Newcastle) and Taiwo Awoniyi (Coventry City) claiming that dubious honor instead.

The curious thing is that, in the end, all of this matters less than you might expect on the scoreline. Mohammed Kudus (Tottenham) has racked up 38 offsides compared to Rogers' two, with an almost identical number of expected offsides, but when you calculate the true impact (the actual skill, the volume of risky passes each player receives, and how many of those lead to goals), the gap between the best and worst in the league at this skill shrinks to barely half a goal per season. Even Haaland, who combines elite ability to stay onside with a massive volume of risky passes, would only add 0.63 goals extra to his season because of it.

Looked at another way, though, that number isn't so small. Over the past decade, each extra unit of goal difference has cost a Premier League club, on average, 2.3% of its wage bill. That means a giant like Manchester City, with a payroll that according to Deloitte exceeds $500 million, would pay roughly $12 million just to add one extra goal of difference over a season. For a single skill belonging to a single player to be worth the equivalent of $7.5 million a year isn't bad at all, especially when it comes paired with other talents like knocking center backs around like bowling pins or finishing with a scissor kick in midair.

The model, of course, has its limits. It can't credit the space a player creates when a defense chooses to mark him tightly, nor can it capture the small tricks that separate the greats: the run made from behind a defender's back, the double fake, the curved run, that habit of hiding behind the center backs just to gain a couple of yards when a teammate takes on a defender out wide. But at least it helps put a number on a skill that usually goes unnoticed, except when the replay gives it away.

The worst thing that can happen to me is being offside, because then you have no chance at all of scoring. But if you're onside, even if you're farther away, you always have a chance.Erling Haaland