Premier League trends, 12 Nov 2018

We’re far enough into the season that it’s worth updating the long-term trend graphics, which show how a club’s results and underlying performances compare over a three-season period. These are explained in full here and briefly below.

Explanation

These are adapted from a very similar design by the Swedish blogger Zorba138 and are intended to track a club’s long-term performance, plus 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

Arsenal definitely look to be running a bit hot so far thanks to some clinical finishing, with overall performances not at the level they were for most of last season. I’m a bit worried about both Brighton and Burnley, whose performances have dipped noticeably, while Tottenham are also experiencing a visible dip (although their habitual overperformance means that results are largely holding steady so far). If Man City get any better then I’m going to need to make the axis labels smaller.