Is Jamie Vardy Actually Underperforming? The Importance of Measurement

Jamie Vardy’s performance has been the story of the EPL season. But how spectacular is his performance? In the analytics community, this revolves around Expected Goal (xG) measures. So let’s look at Danny Page’s great simulator to see how he’s doing.

Vardy

Using @footballfactman’s numbers, Vardy is doing pretty well for himself, finishing 1.64 standard deviations above the mean (only counting non-penalty goals). This means we’d expect him to have more goals than this, given his xG total, about 5% of the time. Very impressive finishing.

But what is 1 xG isn’t really one expected goal? @rakkhis posted a great image today showing the correlation between xG (plus xA, or expected assists), and actual goals + assists. It shows a powerful trend, with an R2¬†of 0.73.

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The interesting thing here is that we see Jamie Vardy, Mesut Ozil, and Romelu Lukaku slightly below the trendline. What that means is that those three players all have expected assists and goals roughly equal to his actual production. In this case, it means we’d actually expect Vardy to have about the same number of goals and assists (likely goals because of his position) that he actually does. This means Jamie Vardy is basically performing identically¬†compared to expectations, which is a remarkable idea.1

This all goes to the importance of measurement: Danny Page’s simulator assumes a 1:1 relationship between xG and actual goals scored. It says that Jamie Vardy is having a rare season, over-performing > 95% of all simulated seasons by someone with his xG. The regression however tells a different story, showing Vardy is slightly underperforming and should have actually scored an extra half goal or so. If we measure it using a 1:1, Vardy is an extreme outlier, if we measure it using the ~1.6:1 relationship that the regression shows, then Vardy is performing almost exactly at expectations. In one version he’s in an amazing purple patch and we’d likely expect him to slip a little bit back to normal, in another, we’d expect him to continue this streak as long as he keeps getting into the same positions to shoot. Measurement matters and it affects the conclusions we can draw from data.

 

  1. EDIT: I originally had this paragraph as “Vardy was under-performing”, but John Burn-Murdoch pointed out that the author of the graph transposed the X and Y axes, meaning people under the trendline are over-performing, while people over the trendline are under-performing. I’ve edited this paragraph to reflect that idea.

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