Thursday 28 March 2013

Finding The Top Corner. A Repeatable Skill ?

Two goals, separated by nine months, but with similar Champions League significance for firstly, Chelsea and then Tottenham. On Monday night*, Gareth Bale picked himself off the floor just as the clock was ticking into the red zone and then produced the kind of dipping, careering shot into West Ham's top corner that is still the exclusive preserve of the game's very best strikers of a football. One point became three and next year's Champions League participation inched slightly closer for this year's Europa League contenders.

Rewind to last May in Munich and the importance of corner kicks to the very best teams is reinforced. Hot on the heels of Manchester City edging the title on goal difference with an overwhelmingly superior corner conversion rate, Chelsea send the Champions League final into extra time when Didier Drogba thumps Mata's inswinging near post delivery into the top right hand corner of the net.

At first glance, two very different goals. There was nothing particularly challenging about setting up Bale with possession in the final third and all of the hard work started as the Welshman drew his foot back just before the strike. In contrast, Mata's delivery to, statistically the most dangerous area of a packed penalty area had to be exact and Drogba then needed to fight his way through a congested box to arrive first to the cross. Once he was in place for the header, a goal was very likely.

However, the goals do share some common ground in that both efforts ended up crossing the goal line at roughly the same point, just below the crossbar and to the left of the right hand upright from the striker's perspective.

Shot placement is likely to be a significant, minor contributing factor to include along with the x, y co-ordinates of the shot origin when estimating the average goal expectancy of a particular goal bound effort. A goal from Bale's position, wide of centre and well outside the penalty box was an extremely unlikely outcome, but intuitively you could assume that by choosing to aim for the top corner, compared to say the bottom corner, the chances of success were slightly tweaked in his favour. Similarly, Drogba's header may well have been marginally more taxing for the keeper had he directed his header downwards rather than at head height. Data collection can only improve shooting models such as these by including shot placement.

If shot placement tweaks goal expectancy defined by shot origin, conversion rates for different areas of the goal will also be significantly affected by position on the field from where such attempts originated. Attempts which are aimed at the top corner will show very different conversion rates if the sample is dominated by "Drogba" type efforts as opposed to "Bale" type ones. We can, however, begin to appreciate the average importance of shot placement by looking at aggregated conversion rates for all EPL sides spread over multiple seasons.

Shot placement conversion rates across different sections of the goal are readily available and can be manipulated to give likely average rates for individual locations. For example a shot along the ground to the centre of the goal to a point where the keeper is highly likely to be positioned, based on the average shot placement success rates for the last four seasons has a generic success rate of just over 10%. If a player manages to scrape the paint of the upright and crossbar with a shot to the very top corner, the average success rate jumps to over 70%.

The data to connect shot placement to shooting distance and angle isn't available, but we can use aggregated data for shot placement and hope that the shooting distances even out in an to attempt estimate the sustainability of the records of individual teams over recent seasons.

Such site as EPLIndex supply success and failure rates for six equal areas of the goal, but the most interesting are the four top and bottom corners of the net. Sample sizes, even for teams are small, so I've further aggregated left and right sided corners for both lower level and higher level shots to produce individual, seasonal team figures for efforts aimed at the top and bottom corners of the goal.

Small sample sizes are prone to large amounts of random variation around the actual talent level that is being measured and even by aggregating data the maximum number of shots aimed at an opponents top right and left sided corner of the net only hits highs of 40 over a season and drops to near single figures for some. Toss a fair coin once and you are guaranteed to be as far away from the true likelihood of a head appearing as it is possible to be. You'll see 100% or 0% heads compared to the true likelihood of 50%.

So we should expect poor converters of top corner chances to appear by random chance in such limited sample sizes, even when the true abilities of sides are closely matched. In my four season sample, Chelsea recorded a league highest 73% success rate with top corner shots during one season and Sunderland managed a low of 15% in another.

We need to try to know if the spread of the conversion rates seen by all Premiership sides over a four year period is characteristic of just randomness or is the spread significantly wider, possibly indicating an element of repeatably skill at work.

Maybe surprisingly, for shots aimed at the top corner there is no evidence that the spread of conversion rates differs much from those expected by chance if each side had league average conversion figures. Sunderland's 15% season was just a league average 43% side getting unlucky, the Wearsiders jumped to 63% in a subsequent season.

No side recorded consistently above or below average conversion rates for top corner shots over four seasons, again leading to randomness as the main cause of differing conversion records across teams. If you wanted to forecast a team's record in subsequent years, the league average would be a better indicator overall than their previous, individual achievements.

For shots aimed at the bottom corners, however, there is strong evidence for a substantial component of repeatable skill, based on the actual distribution of conversion rates. In addition, both Manchester clubs are above average over all four seasons. Along with Arsenal and Chelsea, they also occupy the top six performing seasons, until the almost inevitable arrival of Stoke City. Now if you wish to project conversion rates for bottom corner attempts, league average and past, actual team performance should be included.

Converting shots is a skill, but why converting in different areas of the goal a more repeatable skill compared to other targets will have to wait for another post.

* out of date reference because I wrote half of this post last month :-).

1 comment:

  1. Mark, even if you had all the end locations of each individual shot how do you think you could incorporate that into the likeliehood of a given shot scoring? This, I think, is the where you are trying to end up.

    Don't forget that a shot off target was (presumably) supposed to be a shot somewhere on the goal.
    So even if Chelsea had 73% of their top corner shots end up as a goal, we do not know how many off target shots were also supposed to end up in the top corners. Therefore (even where the end location of each shot is known) I don't see how we can know the success rate of a shot that was aimed for the centre / top corner / bottom corner [delete as appropriate] - which is what I think the article is trying to get at.

    Have I missed something?