Premier League trends, 23 Jan 2018

Now that we’re over halfway through 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

Chelsea look to have recovered from their early dip in performances and are looking better than ever at the moment, but there’s plenty to be concerned about for a Man Utd seemingly moving in the opposite direction in spite of their results. The sheer amount of blue on United’s graph reflects what I spotted in the scatter graphics a few weeks ago.

Both Liverpool and Man City have consistently performed at a high level this season, with their respective hot scoring streaks never looking all that sustainable (but probably not necessary anyway given how well they’ve been doing).

Brighton look to be getting steadily worse, although their recent results have been a bit unflattering. Stoke and Southampton are in the same boat, while the decline at Newcastle has been far less subtle.

West Ham have enjoyed a dramatic revival under David Moyes but they never looked as bad as their results suggested anyway, plus their underlying performances haven’t actually shifted all that much. West Brom also look to have enjoyed a genuine new manager bounce under Alan Pardew, with far more of an impact on the quality of play.

Sam Allardyce’s arrival at Everton also saw an improvement in results without moving the needle much performance-wise. The departure of his reported rival for the Goodison Park hotseat – Marco Silva of Watford – actually looked to be doing pretty well before their recent decline.