Tuesday 27 August 2013

Stoke 2 Crystal Palace 1. A Shot Analysis.

Crystal Palace's visit to the Britannia Stadium pitted together two of the favourites for a return to Championship football in time for the 2014/15 season. As one of the promoted sides, Palace's position is understandable, especially with the loss of Zaha to the champions, Manchester United and an elevation that came by way of the playoffs. Their chance of an immediate return is currently rated around the 70% mark.

The experts are less confident in the prospect of Stoke slipping back down the pyramid come May. In contrast with the pessimism surrounding Saturday's recent opponents, the Potters currently enjoy the considered consensus of a 75 to 80% chance of remaining in the top flight. That number still makes them fourth favourites to be one of the three relegated sides. A sign of the uncertainty surrounding a new manager in Mark Hughes, a seismic change in style and the competitiveness of the lower reaches of the Premiership, rather than an acknowledgement of Stoke as an established EPL team.

So the match on Saturday was an early season opportunity for potential strugglers at the lower end of the table to pick up valuable points, while guaranteeing none for their likely rivals. A nervous 90 minutes awaited both sets of fans, especially Stoke's, who as the home side were reasonably strong pregame favourites and had a manager whom, as a high profile pundit explained "hadn't won a Premiership game since January" (total sample size, one match, away at Anfield).

With the week's marque matchup taking place on Monday night at Old Trafford and the remainder of Saturday's Premiership entertainment providing less than riveting viewing, Stoke's 2-1, come from behind victory appeared second on Match of the Day. As is usual in such truncated highlights, most of the action featured the goals and various goal attempts from each side and initial impressions appear to show that Palace might consider themselves unlucky losers on the balance of attempts.

                                                 Stoke Attempts on the Palace Goal.

                                                 Palace Attempts on the Stoke Goal.

Both sides had 14 attempts on goal with each side requiring the other to make an intervention to a goal bound shot on 5 occasions. So was Stoke outscoring Palace by 2 to 1, not only an absurd use of ratios, but also a fortunate outcome for The Potters and would a share of the spoils been a more appropriate final outcome?

The use of shots and headers to determine the fairness of an actual result is rife with flaws and contradictions. A side tailors their attacking approach to the current game state and that usually means more goal attempts when that state is an unfavourable one. So shot numbers are a function of what actually happened and in which order, on the day. Also a different outcome for one attempt would inevitable lead in reality to later chances being stillborn and others being spawned.

Nevertheless, it  may still be a fruitful and illuminating task to assign each actual attempt a likelihood of it resulting in a score, once factors such as pitch position for the shot, if it was with the boot or the head and if, as was the case for Palace's goal, if it came via a counter attack (or aimless punt down field as it was know when the tactic was extensively used by Pulis' Stoke) are accounted for.

Most Likely Scorelines Given the Quality and Quantity of Palace and Stoke City's Goal Attempts.

Shot Generated Scoreline.
(Stoke Score Appears First).
% Frequency.
1-0 25.2
0-0 16.2
2-0 15.5
1-1 11.7
2-1 7.3
0-1 6.7
3-0 6.2
3-1 2.7
1-2 2.1
0-2 1.4
2-2 1.3
4-0 1.3
4-1 0.6
3-2 0.5
1-3 0.3
5-0 0.1
2-3 0.1
4-2 0.1
5-1 0.1
0-3 0.1
3-3 0
5-2 0
0-4 0
4-3 0
2-4 0
6-0 0

* the more unlikely outcomes occurred with such infrequency that they fail to register to one decimal place.

Just over half of Stoke's attempts came inside the box compared to just less than half for their visitors. So initially, even with such a rudimentary classification of attempts, the home side grabs a slight advantage. Better still for Stoke three of their eight penalty area attempts came in an area bounded by two perpendicular lines drawn from each goal post and three others were only marginally outside this lethal shooting area. Once shots from inside the area move towards the lateral extremities of the box, the likelihood of a score begins to fall away. Palace didn't manage one shot from the more profitable central positions.

As a consequence, although numerically equal, Stoke's attempts had a much higher likelihood of a goal resulting from their efforts. The Potters' goal expectancy from their 14 attempts eclipsed that of Palace's 14 efforts by over eight tenths of a goal.

This goal expectancy supremacy, based on the quality of the attempts made by each side on the actual matchday begins to validate the actual one goal margin by which Stoke won the game. But we can go further by simulating the actual outcome of all 28 shots in thousands of possible iterations of Saturday's match. The excitement may be too much for committed Stoke ( ) and Palace (  ) fans to bear!

Over ten thousand iterations, Stoke were by far the most likely winner, judged solely on the quality shot count. They "won" 60% of the simulations, drew 29% and only lost 11% of the time. The five most likely match outcomes all saw Stoke avoiding defeat and the game's actual scoreline was the fifth most likely simulation scoreline, occurring just over 7% of the time. City fans will hope the 25% chance of such a shootout finishing 1-0 doesn't herald a return to the dark, unappealing days of Stoke's binary past.

Overall, less than sparking entertainment, but a fair result in hindsight and three points for Stoke towards their customary 40 point target.

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