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.

Thursday, 9 April 2015

Deflecting the Blame.

Despite making just two appearances for Stoke City, both from the substitutes bench, Souleymane Oulare's contribution to the Potters' eventual return to the top flight was hugely significant.

His second and final appearance came twenty minutes from the end of normal time in the second leg of the Division Two playoff semi final against Cardiff at Ninian Park.

As had become traditional, Stoke were making a ham-fisted attempt to reach the final. A 2-1 home defeat, combined with a goalless second leg as the game entered the final minute, meant that elimination beckoned.

Cardiff's PA had just urged their fans to remain in the stands to allow a lap of honour from their soon to be victorious team, when Stoke, quite naturally, scored to take the game into extra time.

Five minutes from penalty kicks, Oulare claimed the winner in bizarre circumstances. James O'Connor's free kick was arcing gently into Neil Alexander's left hand corner and the waiting arms of the keeper, when the ball struck Oulare's backside and rolled gently into the opposite side of the net.

Stoke were into the final, ironically to be played at hospitable surroundings of Cardiff's Millennium Stadium, where even the 12 game losing curse of the South dressing room couldn't prevent them from defeating Steve Coppell's Brentford.

Had x, y co ordinates been readily available in 2002, Alexander would have been "guilty" of conceding a speculative 1 in 25 attempt, whereas the reality of the deflection made the shot almost impossible to second guess and save.

Typically around 2% of on target shots take some type of deflection and although the experience of Alexander was extreme, the frequency at which keepers are seen to flail desperately as shots loop unexpectedly from their intended flight before settling in the net, implies that such shots are more difficult to save.

Using data from the recent Premier League seasons, it appears that a third of deflected efforts defeat the keeper's best efforts to save the attempt. Therefore, whether or not a shot was deflected would appear to be a significant additional factor, alongside more usually recorded x,y coordinates and shot type, in determining the likelihood that an effort will result in a score.

A baseline figure for a shot from outside the box increases the likelihood of success from around 0.05 to over 1 in three depending upon the severity of the deflection.

Therefore, a keeper who finds himself facing a disproportionately large number of deflected attempts, may appear to greatly under-perform against a basic shot model that does not include deflections, and their impact on goal expectation for such diverted efforts.

Villa's Brad Guzan faced at least 10 such shots in 2012/13, although the following season was kinder to him. Using combined data from both seasons, a shot model that omits deflections as a predictor, suggested that a single goal was the goal expectation for all deflected attempts faced by Guzan.

Brad Guzan needed all the help he could get to deal with deflected shots in 2012/13.
However, if we re-model all shots to include to added difficulty of saving a deflected effort, Guzan would now expect to concede just over four goals from deflected attempts.

In reality Guzan conceded seven goals from shots that were deflected. An under-performance in both cases, but the keeper's actual concession rate is less damning when we acknowledge the possible effect of a stray deflection may have had on his chances of making what may have been a much easier task.

In a similar vein, Jussi Jaaskelainen over the same period, faced deflected shots that were "worth" 1.7 expected goals from a model that excluded deflections as a variable, but rose to just over 6 expected goals when deflections were accounted for.

He conceded five times from such shots, greatly under performing against a model that fails to differentiate between deflected and non deflected shots, but slightly over performing against a more detailed one that does account for the increased difficulty of saving a deflection.

Repeatability is the key to use data to evaluate players, but a keeper to whom fate hands a glut of deflections to deal with also deserves a model that attempts to allow for the increased difficulty of the tasked he was faced with.