League 1 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

Scunthorpe are fascinating – once again they’re massively overachieving relative to their underlying performances (or at least as well as I can measure them). Despite a worrying drop in their expected goal difference their results have picked up once again, but the bubble could burst just like it did in the second half of last season.

Bradford also made me suspicious in the early part of the season with some seemingly fortunate results, but they’ve rapidly come good even if their form has been less convincing recently.

The turnaround at Oldham has been really impressive and their recent drop in form looks to have been somewhat unlucky, so they should be fine if things continue as they are.

Plymouth‘s results turned a corner before their performances did and their recent results set an unsustainable-looking precedent, but regardless they’ve undergone a rapid transformation after their poor start. Gillingham have also recovered pretty dramatically, but like the Pilgrims they don’t look quite as good as their recent results so could cool off once more.

It’s been a frustrating season for Wimbledon, who have seen results consistently fall short of some pretty respectable performances so far.