League 2 trends, 21 Jan 2018

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

Barnet look to be in real trouble, with a decent start having sharply given way to some consistently poor performances. Exeter‘s own bright beginning to the season has also ebbed and it’s not clear that they’ll be able to sustain a promotion bid.

Mansfield have enjoyed better fortunes of late but these don’t seem to be performance-driven and they could therefore also fall away, leaving them short of pre-season expectations.

Two clubs who are impressing are Wycombe – sustaining a promotion pace with a remarkable level of consistency – and Swindon, who have continued to improve even if results have gone against them lately.

For Forest Green and Lincoln there’s no expected goals line for last season as the National League stopped collecting the necessary shot data. On the subject of Forest Green, their recent uptick in results seems to be at odds with their underlying performances, so it’s still too early for optimism.