A brief note on xG and low value shots.

So I was thinking the other day about xG (expected goals) and how match xG totals can be a little misleading. For instance, if two teams had the same total match xG, but one through many low value shots, whereas another took a few high quality shots, simply stating the xG sum implies the teams were about even.

However, the spread of xG is broader in the team that took lots of low quality shots.

To illustrate this, I’ve knocked up a quick interactive chart in Tableau:

https://public.tableausoftware.com/profile/bntor#!/vizhome/BoxShots/Dashboard1

(Upon writing this, I’ve learnt that apparently WordPress.com doesn’t support embeds of Tableau)

So in taking lots of shots from poor positions, you open yourself up to a greater degree of variance and therefore a larger degree of luck.

So which is better?

Well, unfortunately, my Tableau skills aren’t good enough to incorporate various other metrics/measures into the viz. The good news is that in a lucky bit of timing,  Danny Page tweeted about his xG match simulator (which is of course, what motivated me to write up this post):

https://twitter.com/DannyPage/status/523137736664821761

https://twitter.com/DannyPage/status/523138151615709187

So according to this sim, some xGs are more equal than others;  pay attention to where a team’s xG comes from/how it’s distributed.

This does not, of course, mean that you should only shoot from great locations, but a reminder that we have to keep a balance. While Danny Page’s simulation suggests convincingly that teams should bias their shot selection towards high value opportunities, by cutting out all low-medium value shots, you insulate yourself from the effects of both good and bad luck.

For instance, if a player has the ball in the opponent’s half with little support late on in the game (or you are Cristiano Ronaldo), it may make sense to shoot, since the cost of losing possession is low and the alternative options are probably unlikely to result in a goal. In other words, there is a low opportunity cost associated with the long shot. If, however, you are 40 yards out collecting a rebound from a corner with plenty of teammates in support, it’s probably not a great idea.

Therefore, I would suggest that as long as the cost of your lower value shooting is minimised with respect to high value opportunities (looking at you Andros Townsend), then varying the pattern of your attack with some lower value shots, like headers and long shots is certainly a good thing. If nothing else, it makes you less predictable and forces the opponent to defend in a greater variety of ways.

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