Premier League trends, 9 Feb 2020
As we’re a decent way into the new season, I thought that it was worth updating the long-term trend graphics to see how each club has fared in recent seasons. These are explained in full here and briefly below and this year I’ve expanded the data range from three seasons to five to get a fuller picture of how each club’s performances have evolved.
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.
You can see immediately why Arsenal changed their manager – their underlying performances (red line) have spent most of this season in negative territory. Leicester‘s hot streak looks to be over and their underlying numbers have ebbed slightly too. Serial underachievers Southampton are enjoying their best run of form – both actual and in expected goals – in three years. Jose Mourinho hasn’t yet returned Tottenham‘s performances to their late 2018 peak but things have improved since their poor start.