An Introduction to Expected Saves (xS)

By Raven Beale – @sbourgenforcer

I’m not sure if someone has already written about this or not. If you have, credit to you!

This article assumes an understanding of the flagship metric of the analytics movement xG (Expected Goals). I would advise reading this, this, and this if you are not familiar with the concept.

What are Expected Saves?

Expected Saves is a metric that attempts to estimate the number of saves a goalkeeper should make based on the quality of chances their team concedes.

The model assumes there is a strong correlation between xG and the chance a player misses a shot. More specifically, the higher the xG, the lower the chance of missing. The model I use relies heavily on shot location, angle, use of head & defensive pressure. All these variables in my opinion affect the chance of a shot being on target.

As xG is essentially a percentage of total chances from which a player scores, we should be able to use this into an equation that allows us to predict the chance of a player missing a shot.

Each shot has two outcomes: on target or off target. Shots on target can then be broken down further into saves and goals.

Raven's article

We already have xG to calculate the chance of a goal (on target past the keeper). We just then need to calculate the chances of a shot being off target to calculate the chance of a save being made:

xSave = 1 – xG – xM

xM is the chance of missing a shot. This is calculated by dividing missed shots by total shots, broken down by shot quality (xG):Raven's article 1.png

I then assigned these xS values to each shot based on their xG value. I then summarised the xS value by goalkeeper (for the Premier League):

Raven's article 2

This article was written with the aid of StrataData, which is property of Stratagem Technologies. StrataData powers the StrataBet Sports Trading Platform, in addition to StrataBet Premium Recommendations.

That’s it. I hope you liked the article. If you have any further feedback or questions, feel free to use the comments section below or ask me on Twitter (@sbourgenforcer).


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