Thursday 20 June 2013

Shot Blocking From Corner Kicks.

Corner kicks are often the life blood for those technically limited sides which use them to provide a rare chance to deliver quality balls into an admittedly crowded penalty box. Space and the ability to create it is a major factor in enhancing the likelihood that an opportunity will be converted and so the congested attacking area that predominates during a corner is less likely to result in clear cut chances being created compared to normal, open play, unless you've specifically built a side to exploit this niche. The largely unpunished defensive grappling of recent seasons during a corner kick only goes to reinforce the difficulties faced by would be goal scorers from this particular set play.

In previous recent posts I've looked at the often quite large differences in the conversion rates of chances that are created by various routes. Shots created by way of a quick counter attack appear to be easier to convert, once shooting position is accounted for, presumably because a stretched and under manned defence is less likely to be able to effectively close down both shooters and passers.

So if shots from counter attacks represent one end of the spectrum for defensive pressure, those attempts made in a crowded penalty area immediately following a corner kick are likely to be consistently attempted under the severest of opponent pressure.

Individual shots can of course be atypical of the general situation. For instance a player may through either luck or good instincts find himself totally unmarked even in the most crowded of areas, but in a reasonably large sample we should expect to see a conversion rate effect due to tighter marking when defenders have time to organize themselves.

As in previous posts I've used shots generated during usual open play as a benchmark figure and incorporated whether attempts were headers or by the boot before adding a variable to represent whether the chance came immediately following a corner.

As with most aspects of football analysis, there is an amount of subjectivity in defining goals scored from a corner. Andy Carroll's towering injury time winner for Liverpool at Blackburn in 2011/12, for example was scored just eight seconds after a Liverpool corner kick and much of the defensive chaos was caused by the presence of a handful of visiting defenders in the Blackburn box. But between the ball leaving the corner quadrant and hitting the net it was touched by a Blackburn player and even found it's way back to the halfway line. A goal from open play or a goal from  corner?

As with shots attempted on the counter, chances from corners do appear statistically different from those made in open play once position is factored in. A shot from the edge of the six yard box is over twice as likely to be converted in open play than during a corner kick. This discrepancy is maintained  as we move further out towards the penalty spot, a slightly greater than 20% conversion rate in open play drops to 10% in the rugby scrum following a corner kick.

Jostling for space and glory.
Similar figures are present for headed attempts. A player again can double his usual chances of heading in a corner kick if he or his team can engineer the type of unhindered aerial effort that is more typical of an attempt made from open play. A limpet like defender, more often with a handful of shirt, typical of the excesses allowed at corners, on average drops a would be scorer's rate from over 20% in the open to just above 10% from the sweet spot of the edge of the six yard box following a corner.

We can begin to see how defensive pressure at corners materially alters the anatomy of shot by looking at the frequency of blocks made in our two different scenarios. An average of around a quarter of all goal attempts are snuffed at nearly at source by defensive blocks. So blocks are a major component of shooting analysis. And as with conversion rates, blocking rates appear to be dependent upon the amount and proximity of bodies in  the box.

A player shooting from the edge of the six yard box can expect to suffer the disappointment of seeing 2 out of every 9 shots blocked by desperate opponents, but this rockets to 2 out of 5 following a corner. Headers follow the same pattern, 1 block in 7 for corners compared to 1 in 14 during open play from the same six yard area, displaying the general difficulty experienced by defenders when trying to block headed attempts.

The excitement often generated by corners is not because of an enhanced chance of converting any opportunity that is created, because on average it is a lower quality chance. But in the opportunity for defenders to showcase their game saving interventions by throwing themselves in front of what, from the viewpoint of shooting position would be considered an excellent scoring possibility in more familiar open play.

Pinball meets football.

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