Expected Goals: A stat to keep an eye on

Editor’s Note: Alex is one of several new writers you’ll be seeing at TSF. We’ll be doing a roundtable to introduce them all in the next day or so, but for now, please welcome Alex Ellenthal to TSF!

Yesterday, over at statsbomb.com, Ted Knutson (@mixedknuts) introduced a new data visualization tool: trend lines.

“Trend lines aren’t groundbreaking visualizations, but they are one of the most useful, powerful ways of displaying recent team performance across a wide array of metrics,” writes Knutson in the article.

One of the key metrics Knutson uses the trend lines for is Expected Goals (or xG), which is perhaps the best estimate of chance quality. xG data tells us what the score should have been based on the chances created. Below you can see how Arsenal’s xG numbers fluctuated over the course of last season, according to Knutson and StatsBomb’s data:

courtesy of statsbomb.com

As you can see, Arsenal fell noticeably in the second half of the season. Knutson explains: “From an xG perspective, Arsenal’s April and early May was one of the worst in the data set...The trend shot upwards from late season trouncings of Sunderland, Stoke, and Everton, but despite a lot of spending recently, Arsenal look far from the ranks of an elite team.”

A three-year snapshot of Arsenal’s xG numbers shows how steep of a drop last season’s slump was from what we’ve seen in the recent past.

courtesy of statsbomb.com

On the image above, the 16/17 season is the data to the right of the second gold line.

For context purposes, below are Tottenham and Manchester City’s three-year trend lines:

courtesy of statsbomb.com

Clearly no one metric tells the whole story, and it’s risky to put too much stock into one piece of evidence, but nonetheless, it’s discouraging that while two of their main rivals trended upwards, Arsenal went in the opposite direction. No matter your opinion of xG, if nothing changes here, I have a tough time seeing Arsenal as true contenders for the Premier League title next season.

It’s worth noting here Arsene Wenger’s decision to switch to a three-man defense that sparked a run of nine wins in the final ten games. I believe sticking with the new formation heading into the upcoming season is the right move. Although as Knutson pointed out, Arsenal’s xG numbers were inflated by wins over weaker opponents we expect Arsenal to dominate, they also beat Manchester City and Chelsea (convincingly) in the FA Cup using three at the back. At the very least, it was refreshing to see a change. Because last season’s late burst is an extremely small sample, I’m looking forward to being able to more confidently assess whether Wenger’s tactical shift is enough to turn Arsenal back in the right direction next year.