Thursday 17 December 2015

Chelsea Win In Cyberspace.

Chelsea's defeat at Leicester on Monday night was hardly made more bearable by their moral victory over the title leaders on a myriad of spreadsheets. It certainly failed to impress Roman Abramovich.

From the context-less reaches of hyperspace to the sports pages of the Guardian, the floundering champions gathered three virtual league points in a hard fought, but decisive, expected goals victory that barely required more than one decimal place to confirm their eventual superiority.

In a straight summation of expected goals it is difficult to find a model that didn't rate Chelsea above Leicester on the night despite the Foxes' 2-1 win.

For those who watched the match (and anyone who quotes expected goals or some such, is automatically assumed not to have bothered), the expected goal figures do not pass the eye test.

Part of the problem may arise from incomplete models.

The game was level until the 34th minute, whereupon Leicester took a lead that they increased, saw it reduce, but subsequently kept.

Leicester had five shots on target spread across the 2nd minute to the 48th and none thereafter.

Chelsea had no shots or headers on target until the 62nd minute and four in total, ending with Remy's 77th minute headed goal.

So a "game of two halves".

Game state, score effects, or how ever you wish to describe them eventually alter a side's approach to the match. Risk, reward subtly change based on score line, abilities and time remaining.

A side chasing a deficit appears to see their chances of scoring reduced by around 15% compared to the same opportunity from a side that has the lead. Possibly due to different levels of defensive pressure throughout the chance creation process.

So Chelsea's chances may not have been as gilt edged as they appeared merely from shot locations.

Also closely related events are not additive and Chelsea's two opportunities around the 62nd minute where close enough to have only reasonably been able to deliver a single goal.

If you include these factors on your spreadsheet, the game remains with Chelsea, but they only win around 25% of simulations, with Leicester taking 20% and avoiding defeat in the remaining 55%.

In Cyberspace no one can hear Mourinho scream. (credit @lubomerkov)
Expected goals is a flexible tool, rather than a true reflection of what the score should have been in the context driven environment of a single 90+ minutes.

It can be used to illuminate the effects of last throws of the tactical dice, such as when Chelsea sent caution to the wind.

For example, once Leicester had a two goal lead we can hazard a guess as to the likelihood that Chelsea's mini barrage of chances could engineer a comeback. Leicester hold on for a win around 40% of the time and draw a similar percentage of simulations, despite not troubling Courtois in their second half display.

We can equally ask how likely Leicester were to score two goals without reply when they were on the offensive front foot during the first 48 minutes and whether that outcome was typical for such a first half performance.

In reviewing a single game exp goals just adds another layer of information, it's as useful or useless as bringing us news about the dressing room psyche or attempting to second guess a manager's in game intentions, however eloquently and subjectively they are presented.

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