Premier League trend update, 6 Mar 2017
I thought that it was about time I updated the long-term trend graphics I introduced at the end of December to see how each club is faring and which might be over or under-achieving. These are explained in full here and briefly below.
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:
- The blue line shows the rolling average of a club’s goal difference over the last 10 league games;
- 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.
There are quite a few interesting charts here, with the dramatic improvement of Manchester United under Jose Mourinho particularly eye-catching. Even when results weren’t going their way earlier this season, performances were still strong (as evidenced by the big red patch). It’s also intriguing how consistently Burnley have overachieved and Crystal Palace have underachieved recently: both streaks have continued far longer than usual and suggest that something else may be at work.