Robin van Persie Really Isn't Afraid to Miss.

A year ago I wrote a couple of posts about the shooting characteristics of Robin van Persie under the general heading of "strikers shouldn't be afraid to miss". The data was from 2010/11, so the Dutchman was still an Arsenal legend, rather than a dangerous opponent from a championship seeking rival who makes a once a season visit to the Emirates stadium.

To summarize the post, when comparing the chances of a van Persie goal effort resulting in a score against location corrected efforts for the other main Arsenal shooters, the Dutchman was clearly superior. However, in terms of accuracy, it was his group of colleagues that were more likely to hit the spot and no matter where the shot originated from, van Persie was slightly more likely to miss the target completely.

I speculated that the rest of Arsenal's strikeforce was taking the mantra of "make the goalie work" to rigidly, taking less risks and more guaranteed applause by failing to shoot towards the less accessible areas of the goal for fear of missing. In short, van Persie's shot placement was more adventurous, more prone to missing the target completely, but ultimately more rewarding in terms of goals scored as a result of this efforts struck towards the periphery of the goal, generally being more difficult to save.

I intended to compare van Persie's shot placement to see if he was indeed working with finer margins, but instead the post became a general look at shot placement as well as shot power on the regularly seen location based shooting models. That post turned up last month as Power and Placement , so now is a good time to revisit the original van Persie venture.

Placement is an obvious factor in determining the outcome of a shot, although the effect of shot power is likely to have a much greater influence on the final result. From data used in the previous link, shots hit from identical pitch positions, but aimed closer to the frame of the goal, increase the likelihood of scoring. But when going from a weak to a more normally paced shot from the same spot, the improvement in expectation is generally many times greater. Shot pace is likely to be a major factor in calculating goal expectancy, but it just isn't generally available, so we will have to include placement alone, largely ignorant of the potential effect of shot power.

Although both keepers and strikers have a "natural" side, it's unlikely, given the numerous body shapes a player has to adapt to get a shot away that one top corner is very much different from the other. Therefore, to see how van Persie's shot placement may have enhanced his performance compared to a shot location based model alone I summed his total top corner, bottom corner and central attempts and saw how they compared in terms of frequency to those of similar contemporaries, namely the top scorers from all the other Premiership sides from 2008 onwards and his fellow Arsenal strikers over the same time scale. This approach also boosts sample size.

Percentage of Shots Hitting Particular Areas of the Goal, 2008/13.

Player/Team.	Top Corners.	Lower Corners.	Top Central.	Lower Central.
R. van Persie.	15%	54%	8%	23%
Arsenal exc. RvP.	12%	50%	7%	31%
League Ave. Top Scorers	13%	50%	8%	29%

The impressions from the original post appear to be confirmed by more granular data. Robin van Persie does appear to find the top and bottom corners proportionally more often than his now former goalscoring teammates, as well as beating the pooled average of the top scorer from each permanent member of the Premiership club since 2008. Only Villa's cumulative pool of leading scorers has bested van Persie's ability to hit the corners over the last five completed seasons. For those fond of anecdotal evidence, the Dutch striker has also hit the woodwork 17 times in the last two seasons, perhaps indicating that he is likely to be shooting with an extremely narrow margin between probable glory and individual shooting failure.

By daring to miss, the premise of the original post appears sound, RvP seems to be boosting his scoring rate at the expense of his accuracy.

van Persie outperforms a simply model based around just shot location. If we then incorporate shot placement into an updated version and van Persie (or any other player) consistently outperforms that version, we need to entertain the idea of including other variables, such as shot power. And then onto defensive pressure or player movement and quality of assist. Models always fail to include every possible factor, but as long as we know what is included, they remain valuable. What's absent is the likely cause of discrepancies and this can lead to fruitful, further investigation.

Perfecting shooting models is an onion that's going to take some peeling.

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 :-).