Friday, 28 April 2017

Arsenal's Shooting Accuracy, Nothing To See?

Expected models come in many shapes and flavours, using a variety of inputs and variables, but they almost all relate to goals scored or conceded.

Early expected incarnations also looked a similarly binary events to goals, such as whether an attempt was on target or blocked, but they've never really grabbed the limelight like their bigger, expected goals older brother.

The methodology is essential the same as that used in building an expected goals model.

Variables are the usual mix of shot location, type and attack classification, tested on out of sample data.

Here's the expected number of shots on target compared to their expectation for the top six teams up to the beginning of this month,

The data has been taken from the Infogol App.

It appears to give a fairly straightforward narrative, based around a relatively accepted family of metrics.

Without running a few simulations it would be easy to categorise the six best team in the Premier League as being either wasteful with the accuracy of their attempts (Manchester City) or more than a little pleased with themselves (Manchester United, perhaps slightly surprisingly).

What seems self evident is that there's nothing really to see with the remaining quartet.

Chelsea, at the time had six more shots on target than their expectation, albeit from the smallest sample size, very similar numbers to Arsenal's slight under performance and well in line with Liverpool and Spurs.

However, a cursory look at expected figures verses actual achievement to label a side either under achieving/unlucky or over achieving and fortunate often fails to reveal nuances in a side's scoring or shooting profile that is present in the data.

The only team that deviates significantly from the expected SOT model in the above table is Arsenal, despite masquerading as a side mildly under performing when it comes to working the keeper.

Arsenal are actually pretty poor at hitting the targets when taking shots that the model deems more likely to miss (usually longer range efforts). While they are a lot better at hitting the target with attempts that the model has decided are much more likely to be on target and require a save.

These two fairly large deviations from the predicted arc of expectation from the model at opposite ends of the likelihood scale roughly balance out giving Arsenal an actual accuracy figure that comes close to matching their predicted figure.

In short, Arsenal may look fine in the aggregate, but they're the only top six team that have individual ranges of actual outcomes that deviate by an interesting amount from this decently robust model for the Premier League.

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