Now that we’re roughly halfway through the season, I thought that it was about time I updated the long-term trend graphics to see how each club fared throughout their last two-and-a-bit 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

Just to pick out a few that caught my eye, Brentford have long been the darlings of expected goals models with strong underlying performances, but the bubble has well and truly burst this season. The departure of possible sorcerer Mick McCarthy has seen Ipswich‘s long tradition of blue-shaded overachieving come crashing to an end. Leeds may have been running hot at the start of the season but they’ve gone from strength to strength while fellow promotion challengers Middlesbrough – another hot starter – have undergone a mini-crash. Norwich have also exploded into life after an indifferent start to the season, while the exploits of Nottingham Forest and West Brom haven’t quite been reflected in their performances (at least as well as I can measure them).