Thursday, 31 January 2019

A Non Shot Addition to the xG Family

Shot based expected goals models can tell us a lot about a match by extending the sample size from around three for actual goals to well into double figures for goal attempts.

But they are event based descriptions of a match and don't always tell the whole story of a match.

The weakness of event based models, be they attempts, final third entries or touches in the box, is, rather obviously, that these event have to occur for them to be registered, often in the most competitively contested region of the field.

Non shot xG models can fill the void that sometimes exists by examining such things as possession chains and the probabilistic outcome that may occur between two teams of known quality.

Last night Liverpool drew 1-1 at home to Leicester.

The hosts, depending on your view point, were unlucky to lose because, "Leicester defended well", "Atko reffed the game poorly" or "Liverpool weren't themselves".

Shot based xG universally gave the match to Leicester. They created better chances and had a larger total shot based xG than the title contending Reds.

Here's Infogol's shot map from last night. Leicester created a couple of decent chances. Liverpool were restricted to attempts from distance.

However, if we look at the potential return for each team based on where and how frequently they began attacks against each other, combined with the typical outcome of such possession in expected goals terms and the talent based differential at completing or supressing passes or dribbles, the balance of "probabilistic" power shifts.

Liverpool shaded the non shot xG assessment by 2.4 to 1.1.

They had the ball frequently enough, beginning in sufficiently advanced areas to have scored a likely two or three goals, with a penalty thrown in for good measure.

Leicester would have typically replied once.

So why was it just 1-1.

Just plain randomness ? An early goal that caused Liverpool to cruise somewhat in a similar way to the return game earlier in the season. A clever Leicester game plan that frustrated Liverpool with a packed defense and a bit of luck from the officials.

There's no correct answer, but there are tools, both event and possession based that can add clarity and suggest areas of investigation.

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