One of the big open questions in expected goals research is accounting for individual finishing skill: a shot taken by Lionel Messi is worth more than a shot taken by Jesus Navas, but how do we account for that? There are any number of open issues here, mostly methodological, but I’ve recently started thinking about a more theoretical one that should be addressed before worrying about the statistics underlying the concept: What exactly do we mean by “finishing skill?”
As far as I’ve read, finishing skill is typically thought of as the ratio of goals above (or below) the number of expected goals. Dismissing the ideas of variance and imprecision in measurement for a moment, a player who scores more goals than expected is a good finisher while a player who scores fewer goals than expected is a bad one. Lionel Messi outperforms his expected goals, so we can say that he’s clinical in front of goal, while Jesus Navas underperforms so we can say that he’s…well as a Manchester City supporter I don’t want to talk about it. But is this all there is to be said?
One of the great contributions of expected goals is the idea that all shots are not created equal, that is to say a shot taken from out wide and from outside the penalty area is less likely to score than one taken from the center of the goal six yards away. But why shouldn’t we apply this to the idea of finishing as well? I’m coming around to the idea that there are as many types of finishing skill as there are types of shots, so the next step is to identify the important ones and identify which players are what types of finishers. To do this, I draw on some of the fundamental contributions of expected goals research along with the eyeball test from watching and playing however many thousands of hours of soccer.1
The Clinical Finisher
This archetype comes from the idea of players who are clinical in front of the net. They’re calm, collected, and don’t miss easy opportunities. In xG terms, they score on high probability shots even more frequently than one would expect. A 0.5 xG shot kicked by Lionel Messi one-on-one vs. the goalkeeper is more likely to go in than one kicked by Fernando Torres at Chelsea. So theoretically that shot is 0.7 on Messi’s boot and 0.3 on Torres’s. This likely correlates with things like confidence, composure, and close ball control.
The Long Range Sniper
Shots taken outside the box and from an angle have a low xG value, yet some players continue to take them. Presumably some are better at these shots than others, and being able to shoot from distance is certainly a skill that can be developed. We could also expect some players to not realize that they don’t have this skill and still take a number of shots from distance, so we’d see some significant variation here. I’m thinking about Zlatan’s famous bicycle kick for Sweden: for a mortal man that shot would be 0.00001 xG, but for Zlatan maybe he makes that as many as 1/10 times (0.1 xG).
The Free Kick Specialist
Direct free kicks are a skill some players have while others don’t. Players like Cristiano Ronaldo, Yaya Toure, or Andrea Pirlo probably deserve a decent bump in xG from direct free kicks, while others players are probably below average. There’s some difficulty here in that only good free kick takers would really ever take any, but it’s a skill we could measure.
The Head of the Class
We know headed shots are lower value than those that are kicked, but obviously this isn’t equal across the board. Andrea Pirlo has never been known as a great header of the ball so maybe a header taken by him that would normally have an xG value of 0.4 would have a true value of 0.3, while someone like Zlatan or even Gerard Pique would have more talent in this area and would be worth a 0.5.
There are likely more types – players who are better on counter attacks, players who are better on corners, etc., but I wanted to present a few basic archetypes because it’s worth discussing and worth thinking about not just finishing skill, but types of finishers. If you’re trying to build a team, you wouldn’t just want the best finishers using the pure xG/actual goals metric, you’d want complementary players. Maybe you’d want to build a team filled with speedy, clinical players who could finish goals on counter attacks. Or maybe you want to play along the flanks and cross the ball into the box 30+ times a game, so you’d want some forwards who are strong headers of the ball. Maybe every team needs at least one free kick specialist, or maybe you’d want a balance. But regardless of the strategy, using xG to define different types of finishers would be a useful addition to the toolkit.
- All players and numbers I use here are hypothetical. The point isn’t the identify specific players or specific values, just to present illustrations of what I’m thinking ↩