Premier League trends, 30 Nov 2017

Now that we’re a decent way into 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

Each club is listed in alphabetical order and I’ve picked out a few observations to give you the general idea. Man City‘s performances have unsurprisingly found another gear after their summer investment. The sheer extent and consistency of Burnley‘s ongoing overachievement under Sean Dyche never fails to impress me – it’s been going on for so long that I’d say he’s definitely not just riding his luck. Crystal Palace meanwhile have been significantly underachieving this season and never really looked that bad, so a recovery seems likely. Newcastle look to be fading after a promising start, while Swansea‘s dip in form doesn’t surprise me given how much worse their performances were than their early results.