League 2 trends, 2017/18

Now that we’re at the end of 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‘s recovery after a poor start was far too sluggish and some wasteful finishing meant that they couldn’t capitalise sufficiently on their improved performances to escape the drop.

While Exeter finished the season strongly, they’ve been overachieving on some modest performances this season and could struggle in the play-offs.

Grimsby were never as bad as their league position made them look, so their late dash for safety feels like an overdue reward. However there’s still work to do if they’re to return to last season’s performance levels.

Notts County looked strong in the first half of the season but performances have waned and they’ve limped over the line somewhat. The same can be said for Yeovil, who have finished a season poorly for the second year running.