Pages

Monday, 11 May 2015

Middlesbrough's £10 Million Corner Kick.

Jose Mourinho isn’t a fan of corners, apparently. So he no doubt took the opportunity of such a 94th minute set piece at Griffin Park to slip away and beat the traffic following Middlesbrough’s first leg Championship play-off game with hosts Brentford.

Middlesbrough boss, Aitor Karanka, perhaps takes a more positive view of the much maligned corner kick than his friend, Mourinho and he was rewarded on Friday night with an injury time, deflected winner from Spanish defender Fernando Amorebieta.

The value of a corner kick is still debated, but for this particular effort we can quantify it in plain GBP. Prior to the injury time kick, Middlesbrough had around a 65% chance of making to the play-off final, reputable now worth around £130 million, for which they will be around a coin toss to reach the Premier League.

After the late winner, they’ve advanced their chances to round 82%. So in virtual, probabilistic currency, the corner, or more accurately the goal from the corner, has increased the value of Middlesbrough’s chips by around £10million.

Of course the ultimate result is an all or nothing outcome, with Brentford and either Norwich or Ipswich still very much interested parties.

More generally, corners result in much less extravagant potential swings in fortune. To take 2012/13 as a typical Premier League season, teams scored an average of 7 goals in the season from corners, compared to 46 from all other means. 

So goals from corners accounted for 13% of all goals scored.

Averages inevitably hide the extremes, only 2% of Newcastle’s goals were from a corner, followed by 6% of Arsenal’s, while Stoke scored 8 such goals and that made up nearly a quarter of their total scores.

Execution also appeared to vary, Arsenal needed to take 260 corners to score precisely half the total goals scored by Stoke from almost exactly 100 fewer set pieces. The Gunners also fashioned just 61 goal attempts from their 260 corners, again compared to 60 by Stoke from only 163 flag kicks.

The style of Stoke, pre Hughes, bore little resemblance to most other Premier League sides, especially Arsenal, but we should still entertain the possibility that the spread of chance creation and goal scoring from corners, seen at its most raw between these two rivals, may be simply due to random chance, rather than differing levels of talent or intent.

Overall 3.2% of corners resulted in a goal in 2012/13, but as we’ve seen there were extremes. Newcastle scored from just 0.5% of flag kicks, Arsenal, 1.5%, Spurs and Swansea, around 2%, climbing to nearly 7% for Manchester United, 5.5% for Wigan (who won the FA Cup with a last minute goal from a corner) and 5% for WBA, Chelsea, and Stoke.

Swansea take a corner.....Don't hold your breath.
This spread of conversion rates does suggest we are seeing something in addition to random variation. And it is repeated if we further look at the rate at which teams muster chances from corners. Arsenal fared the worst, creating an attempt from 20% of kicks and WHU were best with a 35% rate of diverting the ball goal-wards.

Shot models also suggest that a typical team would have an average 12.2% chance of converting each opportunity created by Stoke from corners in 2012/13, but just a 9% chance from each of those fashioned by Arsenal.

It is hardly ground breaking, but the evidence suggests that in 2012/13, Stoke were better at creating better and more plentiful goal scoring chances from corner kicks than were Arsenal and that graduation of skills probably existed in the remainder of the Premier League.

Unsurprisingly, the reverse is true of open play. 

There is no comparable precursor to open play chances as there is for opportunities made corners, final third possessions would probably come closest if it was readily available. The efficiency with which final third incursions are turned into open play goals may give a fairer comparison for corners to be judged against.

On this occasion, Arsenal beat Stoke hollow. They created 474 open play chances, from positions which would give an average side an 11% chance of scoring, compared to the Potters’ 236 open play attempts, each with an average generic 9% success rate.

So, in the grand scheme of an entire season, corners, as a means to create chances and ultimately score goals were a big deal to Stoke.  The Potters scored a quarter of their goals from this method and they were among the best in the league in terms of creation and conversion rates. A third of the Potters’ 25 best scoring opportunities were also made from corners.So they were a rich source of big chances during the Pulis swansong.

Sides which appeared to have both a talent to over perform against the average from corners in 2012/13 and gained the largest proportion of their total goals from such a source, included Wigan, who were ranked 2nd in over performance and 3rd in terms of goals from corners as a percentage of total goals. They were followed by Chelsea, Manchester United, Sunderland and Stoke.

Sides for whom corner kicks and their outcomes were largely an irrelevance, included Arsenal, Newcastle, Spurs and Swansea.

Soccer is a low scoring sport and even at base rate conversion levels for corners, the resulting goals from an average of 11 events per match account for 13% of a game’s total goals scored and when a side, such as Stoke can increase these baseline numbers, the importance of corners to them increases.

Crudely removing Stoke’s scoring from corners in 2012/13 leaves them marooned on 36 points, the same as the final relegation spot, rather than safe in 13th spot.

Not all corners by circumstance produce £10 million shifts in fortune, but whether they are the icing on the cake, as in the case of Manchester United in 2012/13 or a means to survive, as in Stoke’s case, they are important footballing events. 

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.