Premier League trends, 2017/18

Now that we’re over halfway through the season, I thought that it was about time I updated the long-term trend graphics I introduced last year to see how each club fared throughout their last three seasons. These are explained in full here and briefly below.

Explanation

These are adapted from a very similar design by the excellent Swedish blogger Zorba138 intended to track a club’s long-term performance and whether this was an underachievement or an overachievement based on the balance of chances created.

There are two lines:

  1. The blue line shows the rolling average of a club’s goal difference over the last 10 league games;
  2. The red line shows the rolling average of their expected goal difference, based on the quality of chances they’ve created and faced.

Comparing these two allows us to see not only how a club’s performances have changed over time, but also whether there were any differences between the balance of chances created (a useful measure of underlying performance) and goals scored.

These are shaded as follows:

  • Blue shaded areas are where goal difference is higher than chances created, suggesting an overachievement;
  • Red shaded areas show the reverse, where the balance of chances was healthier than the actual goal difference, signalling underachievement.

Over the long term we’d expect the two lines to converge unless there’s a significant difference in a club’s attacking or defensive skill compared to the average for the division. We can’t tell from the data alone whether skill or luck is the cause, but the longer a difference persists the more I’d suspect the former.

Club-by-club graphics

Burnley‘s graph continues it’s blue streak of near-uninterrupted overachievement versus performances (at least as well as my data can measure them). I suspect that a big chunk of this can be explained by defensive positioning, as when I’ve looked at the StrataBet data for the Premier League the Clarets definitely stand out.

While you’d expect top sides to overachieve compared to expected goals more often than not (due to having players of above-average quality), the extent of Man Utd‘s

Newcastle have recovered pretty well since their low point in the middle of the season, and unlike their recovery under Benitez when they were last in the Premier League they’ve also been able to avoid underachieving.

Southampton have been the anti-Burnley this season: consistently underachieving to the point where you suspect that the data’s missing something. However they definitely look to have turned a corner lately, with performances on the up.

Stoke‘s relegation isn’t that all surprising given that they look to have been getting steadily worse for quite a while.

Tottenham‘s performances have taken a worrying nosedive recently – this could just be the result of injuries but it’s a pretty steep decline.