League 1 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

Barnsley look to be coming down from their early season high in a pretty sharp fashion, so hopefully for Tykes fans they’ll pull out of that dive soon. Bradford’s decline has been a lot more drawn out, with neither actual nor expected goal difference having been above zero since last Spring. Luton meanwhile have barely broken their stride since securing promotion and negative goal difference has become something of an alien concept at Kenilworth Road. Peterborough have been running incredibly hot so far and it’s not clear if their bubble is bursting or if they’re doing something unusual that the data can’t measure. Sunderland are also a bit of an enigma: results have been impressive but performances (at least as well as I can measure them) much less so. The sheer amount of red-tinted underachievement on the Wimbledon graphic makes me feel simultaneously sorry for Neal Ardley and also wondering if there was something systemic going on there.