Friday 28 May 2021

What is Goal Expectation?

Let's say you want to make an informed estimation about the upcoming England vs Scotland game at Wembley Stadium in Euro 2020 (2021).

One route would involve estimating the average number of goals England are likely to score against Scotland at Wembley and the average number of goals Scotland would score against England at the same venue.

You could then take a mathematical route to calculate the probability that two side with these average  goal expectation estimates would result in a home win, away win or a draw.

Typically a Poisson approach.

The average number of goals expected to be scored or allowed by a side in a future game has for over 30 years been referred to as their goal expectation

Unfortunately, a more recent and widely discussed metric based on the chance quality of a scoring opportunity, has arrived on the scene and taken the very similar name of expected goals.

They are not the same.

The former, GOAL EXPECTATION, is a measure of the likelihood of success for a side prior to kick off, based on historical data that is used to quantify the difference in quality between the sides. (It may even use historical expected goals data).

The latter, EXPECTED GOALS, is a value ascribed to the quality of attempts on goal, after the fact, based on the characteristics, shot type, location etc of each attempt.

The goal expectation of England and Scotland in the upcoming game is around 2.12 goals and 0.48 goals, respectively.

The expected goals for the game hasn't yet materialised.

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