Friday 10 August 2012

The Anatomy Of A Spanish Goal.

One of the most exciting and fast moving developments at the present involves goal expectancy calculations and how they can be applied to individual player contributions. I've laid down some initial thoughts and calculations with data kindly provided by OptaPro here. The basic idea involves the increased likelihood of a team scoring based on where on the pitch they have possession and how the credit for that situation could be divided. The use of probabilities instead of actual real life records is attractive because randomness can result in for example goal gluts or famines leading to players receiving unrealistically extreme praise or criticism.

It's inevitably that numbers will be a central part of this type of analysis, but at the moment they should be considered secondary to the methodology. It's fairly easy to create a goal expectancy grid for the attacking portion of the pitch using general accumulated data and observational skills. In general the further away from goal and the wider out you are, then the less likely you are to score. So any pass that moves the ball to a closer and more central position is going to improve the scoring chances of a team.

Below I've taken a more detailed look at the assisted goals scored by Spain in their successful Euro 2012 campaign, other opportunities of course were created but not converted. At the moment I've solely evaluated the position of the ball in terms of how likely a team is to score should they attempt a shot from their current position. This approach isn't realistic for general play, as teams also use possession as a defensive tactic, but it is appropriate for overtly attacking situations.

How Spain Increased Their Goal Threat From Passer to Scorer.

Goal Probability
at Point of Pass.
Goal Prob. at Scorer's 1st Touch. Goal
Prob. at Point of Shot.
Change in
Prob. From Pass to 1st Touch.
Change From
1st Touch to Shot.
Scorer. Passer.
0.103 0.341 0.480 0.238 0.139 Navas. Iniesta.
0.035 0.209 0.209 0.174 0 Fabregas. Silva.
0.004 0.023 0.149 0.019 0.126 Torres. Silva.
0.030 0.062 0.169 0.032 0.107 Fabregas. Silva.
0.023 0.117 0.117 0.094 0 Torres. Xavi.
0.006 0.030 0.080 0.024 0.051 Jordi Alba. Xavi.
0.085 0.155 0.155 0.070 0 Mata Torres.
0.214 0.277 0.277 0.063 0 Silva. Fabregas.
0.098 0.137 0.137 0.039 0 Alonso. Jordi Alba.

The process is best described by an example and I've chosen Jordi Alba's forward burst onto Xavi's pass for Spain's second goal in the final, highlighted in red in the grid. In the first column Xavi has possession of the ball over thirty yards away and to the left of the Italian goal. Had he tried a speculative attempt from that position, typical shot data from Euro 2012 and the EPL suggests that he would score less than once in every 160 tries. So the direct goal threat from this position is negligible. Xavi finds Jordi Alba, who has continued his run with a forward through ball and the soon to be scorer makes first contact with the ball outside the box and still to the left of goal. Had Jordi Alba attempted a shot with his first touch his generic chances of scoring would have been about 1 in 30. So Xavi's pass has advanced Spain's likelihood of scoring from 1 in 160 to 1 in 30. The ball is now firmly at the feet of Jordi Alba and he advances the ball into the Italian box and from the position of his first touch he now has increased Spain's chance of scoring to 1 in 12. And he duly adds to Silva's opening strike.

Broken down in stages we can begin to tease apart the individual contributions made by each player. Italy saw the threat level rise from negligible to considerable through a combination of Jordi Alba's run, Xavi's pass, Jordi Alba's running with the ball and finally his shot and Goal Expectancy can be used to quantify the likelihood of success every step of the way. Similarly, Silva's quick thinking turned a routine corner into a dangerous situation against Ireland, Fabregas then elevated the threat by quickly heading towards goal to complete the 4-0 rout. It may also be noteworthy that over half of the nine chances created by Spain required no extra input from the scorer other than the execution of the shot or header. Silva, Xavi, Torres, Jordi Alba and Fabregas each providing pin point passes that required no adjustment by the scorer.

One small step.............

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