Thursday, 16 May 2013

The Art And Talent Of The Corner Kick.

Stoke City's innovative style of play, involving making the very best of the limited assets available wasn't merely restricted to the Delap/set piece routines so familiar to recent Premiership audiences. Paul Maguire, floating in the near post corners and Brendan O'Callaghan providing the headed goal or a worst the delicate flick on were a staple ingredient of match days at the Victoria Ground in the early to mid eighties. Both are still fondly remembered, especially O'Callaghan, who announced his Stoke City debut with a goal within 10 seconds, as a substitute.....from a corner. However, players move on, tactics change and strategies are developed to combat every successful system and in Stoke's more recent non Premiership past, they were regarded as a side that couldn't buy a goal from a corner. Once back in the top flight, Stoke reacquained themselves with the joys of scoring from set pieces in general and corners in particular.

If converting corners into goals is a talent that is distributed unevenly between teams and therefore, ebbs and flows across the decades, we should be able to see both repeatable team traits across seasons and conversion rates that diverge from those expected if the process was simply centred around the league average in a purely random manner.

The average goal conversion rate from corner kicks in 2011/12 was just over 3%. Highs of 5.5% were seen at Manchester City, lows of zero percent at Villa Park and an average of 200+ corners were attempted per team across the Premiership. Equality of opportunity was guaranteed for each corner at the outset of the kick because they are all taken from near identical pitch placements and the spread of the individual team success rates polarized by City and Villa over the last completed campaign, implies that some teams are more talented corner takers than others. If we attempt to account for the random variation component, we are left with conversion rates that are more indicative of the actual talent of each team and this figure is more likely to predict future performance than the actual conversion rates recorded by a side.

Manchester City were likely to have been good and lucky in 2011/12. So a conversion rate  nearer to 4% than their actual figure of 5.5% should really be entered against their name and likewise a near 2% conversion rate is a more accurate legacy to Villa's corner converting prowess for the 2011/12 season. It is probable that they experienced the perfect storm of being both generally poor takers of a corner and unlucky and improvement through a variety of routes should have been expected in 2012/13.

The eventual champions scored at least one goal from a corner once in every three matches and their closest challengers, United needed on average an extra match to do likewise. Overall, by Opta's definition, two matches out of every seven in 2011/12 saw at least one goal scored from a corner kick situation.

If we move on to the defensive side of the ball, the same effects are seen. The actual observed conversion rates allowed by each defense is more spread out than you would expect if each defence shared an identical ability to defend corner kicks. Also, by dragging extreme results closer to the league average and giving more weight to the raw figures recorded by sides which faced larger numbers of kicks, we produce numbers which are more predictive of future performance.

The poorest five performers at defending corner kicks in 2011/12 occupied the bottom five slots in the final Premiership table. Corner conversion has often been an avenue to excel at on the route to preserving top flight status, but by neglecting their duties at the other end of the field and leaking goals from corners at rates of at least one goal every three games, both Bolton and Blackburn's ultimately suffered relegation. Although Wolves managed to narrowly see off the trifecta, they were only marginally better than Wigan and QPR and also experienced the first of multiple demotions.

Interestingly, the season on season correlation for defensive performance is stronger than the corresponding attacking situation. Possibly the ability to make something happen (score from a corner) attracts more attention than the ability to prevent something from occurring. Therefore proficient corner scoring teams are quickly identified and schemed against in future meetings (Delap's longthrow survived as a potent weapon for barely three Premiership seasons and only remained effective thereafter in the unfamiliar territory of the cup competitions).

Villa employ a novel corner defence by attacking the ball.
Such attributes as aerial ability is an obvious advantage when defending and attacking against corner kicks and there is a weak correlation between corner competence at either end of the field. Above average converters of corners are slightly more likely than random to also be above average defenders of such a set piece. However, the weakness of the relationship hints at the diversity of talents that comprise a successful corner. An excellent delivery, as provided by a Robin van Persie or a Paul Maguire doesn't help his side defend a corner, but a good header of the ball is an asset at both ends of the pitch.

Raw conversion rates can hint at different talent levels of corner conversion and a relatively strong season on season correlation also implies a repeatable skill is present. But a deeper analysis of corner strategy requires isolation of every associated skill, from ball delivery to off the ball running and even the semi legal art of blocking opponents. As a valuable scoring method, a 3% conversion rate may not initially impress. However, as @analyseFooty suggested in relation to this post, if we consider a corner kick as just another pass, compared to an average pass, it is a devastatingly efficient one!*

All data is taken from the MCFC release of 2011/12 data in conjunction with Opta.

*(on average an EPL team makes 450 passes a game and scores 1.3 goals, of which about 70% are from open play. Therefore conversion rate per pass is of the order of 0.2 to 0.3%. In 2011/12 over 4,000 corners produced 131 goals, therefore, conversion rate is around 3%. Even allowing for general passes which don't carry attacking intent and accepting that not every goal scoring corner is a first contact score, corner conversion rates still easily hold their own).

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