Tuesday, 5 May 2015

Do Deflections Help the Best Sides?

Monday night’s Premier League game at the KC Stadium, home of Hull tigers to some, but just plain City to the majority, went largely to expectations.

Thirty three points separated the hosts from visitors, Arsenal and unlike the previous match, when Liverpool made the same trip, league form prevailed.

Ramsey scored the decisive second goal, following an obligatory injury scare, Hull slipped marginally closer to the drop, Arsenal edged nearer to consolidating second and the visiting fans once again baulked at paying £50 a ticket for the privilege of following the evening’s main attraction.

Sanchez and Ramsey, respectively claimed the first and second Arsenal goals, both scored from reasonably enticing shooting positions. But deflections on both occasions rendered Hull keeper, Steve Harper almost powerless to intervene.

Sanchez’ free kick took a 45 degree detour, via the head of Dawson and Ramsey’s goal bound shot looped in off Brady’s trailing leg. Harper had the best seat in the house for each goal and more ever was paid for the privilege.

While single incidents should not automatically validate events as the norm, the ability of deflections to turn decent chances into near cast iron, big chances, was again possibly in evidence. The impact on goalkeepers facing such deflections was looked at here, so now I’ll look at how individual teams have to cope with such unexpected events.

As you’d expect the percentage of shots from a team which take sizeable deflections vary between sides. In 2012/13, Chelsea had over 20 such efforts from their over 600 total shots, just over 3%, whereas Stoke benefited just a handful of times from their nearly 400 attempts.

However, even with Stoke’s percentage deflected shot figures around half of Chelsea’s, there is scant evidence that this is little more than natural variation within a smallish sample of shots.

Manchester City shared top billing with Chelsea, as the teams with the highest rate of deflected shots, but then followed the likes of Sunderland and relegated Reading. Manchester United and Liverpool were languishing among the “unlucky”, percentage-wise.

Chelsea may have been particularly fortunate in 2012/13 and the general trend is that to gain more deflections a side needs to take more shots or headers. Around 2% of your total shots will further trouble the keeper by taking an unexpected detour.

However, sheer weight of shots attempted and allowed by Chelsea, even using the baseline 2% figure will mean that Chelsea will turn standard chances into so called “big chances” more often than will their opponents.

In reality, during 2012/13, sixteen league games were played when Chelsea had more deflected on target shots than their opponents, the situation was reversed in just five games and one match was “tied”.

A rudimentary shot location based model will therefore fail to pick up the higher tariff save required to intercept a shot which begins its life heading in one direction before ricocheting off in a completely different one.

Judged by Harper’s enforced indifference as the ball flew feet from him last night, accounting for the increased likely goal expectation from deflected attempts will produce a different array of possible outcomes, especially for high volume shooting teams such as Chelsea were in 2012/13.

As speculated here, how goal expectation is diced per individual attempt can alter our conclusions in a sport of few scores, such as football.

Therefore, I produced two basic models, one that was aware of deflected attempts and one that wasn’t. I then simulated Chelsea’s 38 game 2012/13 season using the two models. The former greatly increased the number of “big goal expectation chances” created by Chelsea, while the latter produced a broadly similar goal expectation over the whole season, but had fewer big chances.

Disregarding penalty kicks, Chelsea had an attempt which had a likelihood of scoring in excess of 40% an average of once every other game when the potency of deflections were incorporated into the model, compared to half that number in the opinion of the model when that information was not used.

And identifying these “big chances”, even if Chelsea’s percentage was also probably luck driven, as well as volume driven, pushes higher the likelihood of the Blues gaining more than 80 league points in the simulations compared to simulations which fail to highlight this potential source of goals.

On Monday night, Arsenal had 19 attempts to Hull’s five and while Steve Bruce will no doubt feel aggrieved at the deflected nature of Arsenal’s opening two goals, there was around a 34% chance that at least one shot would be deflected during the game, requiring more from his veteran keeper.

Less likely at around 6% was that Harper would have to deal with two or more such unpleasant surprises.

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