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July 23, 2013

The Problem with Shots on Target Metric

By Colin Trainor

Ask anyone to measure the efficiency of a striker today and invariably you will receive a reply which will include a metric that is linked to shots on target.  Of course any such measure will be more accurate than those which are based solely on the number of shots that a player took, however that doesn’t mean that is the best measure by which to measure the effectiveness of strikers.

That opening thought was prompted by the following tweet from Squawka Football that I read this morning

Squawka

So Adebayor had the best shot accuracy, yet he only scored a handful of goals and I didn’t recall anyone last season remarking on well the big forward was striking the ball.  Although 70% of Adebayor’s shots found the target (which is the traditional method for defining accuracy) he was missing some va-va-voom which would have allowed more of those shots to turn into goals.

At this, I dusted down my trusty database and had a look at the shots that Adebayor took last season:

Adebayor Shots

Adebayor Shots

We are looking at the goal from the view of the striker, with the two black rectangles representing the frame of the goal.  The red balls signify shots that resulted in goals, and the white balls are shots that either missed the target or were saved.

Now, we can begin to understand why despite having such a high proportion of shots on target that Adebayor didn’t overly work the scoreboard operators across the country last season.
Virtually all of his on target shots were hit straight up the central channel of the goal.  In fact I only count five on target shots which were directed towards the corners, of which two were goals.

If you read my piece from yesterday you will recall that, all other things being equal, a player can only expect to score from shots which are hit straight up the middle about 12% of the time.  That compares to 50% or more when the shot is tucked right into the corners.
I would contend that the metric of shots on target as the measure of choice for determining player accuracy is flawed such is the rarity of centrally struck shots becoming goals.  I believe it’s time to start thinking of centrally struck shots as relatively bad things, not good things; or at the very least that not all shots on target are equal.

Rafael van der Vaart and Darío Cvitanich

Here are the stats for last season for two players, one of whom is much better known than the other.  We have the Hamburg and former Tottenham Dutch man Rafael van der Vaart and Dario Cvitanich a player without any international caps who plays for Nice in Ligue 1.

VDV Comp

The two players had virtually identical numbers in terms of shots and shots on target, yet we can see that Cvitanich had a conversion record for his unblocked shots which is worthy of another planet.  His total of 19 goals was enough to see him finish as joint runner up to Ibra in the French Ligue 1 goal standings table last season.

Although the two players played in different positions, and thus I have no doubt that the types of shots that each player had were not comparable, both players would have identical accuracy ratings per the current accuracy metric.
Based on this bare statistic, that both had an “accuracy rating” of  approximately 60%, we are left to wonder why one player scored almost four times as many goals as the other.
We might be forgiven for thinking that it was due to variance in that the Van der Vaart faced a series of keepers who had the games of their lives, but that thought is short lived when we actually see the location of where the shots were targeted.

Van der Vaart Shots

Van der Vaart

It’s no surprise to me to see that all five of VDV’s goals were scored with shots
that were aimed towards the sides of the goal and that the whole cluster of shots that
would have hit the goalkeeper were effectively wasted, as not one single goal
came from them.

Cvitanich Shots

Cvitanich Shots

Now isn’t that a pretty picture?

Despite having a similar volume of non blocked shots as Van der Vaart look at how few of Cvitanich’s 47 shots were hit straight up the middle.
Again, I have no doubt that the types of shots taken by the Nice striker meant that it was easier for him to hit the sides of the goals than Van der Vaart.
But this article isn’t written as a downer on the Dutch maestro, instead it is intended to highlight why simply using a shots on target measure to determine accuracy is flawed, especially give the level of data and statistics that are available in today’s football world.

The above is not an isolated case either.

Pog Comparison

As before we can see that the number of unblocked shots and shots on target are similar for Reading’s Russian forward Pavel Pogrebnyak and another relatively unknown Ligue 1 player, Toulouse’s Israeli international Eden Ben Basat.

However, the two players post vastly different goal scoring records from last season, and again the shot placement images for them clearly reveals the reason for this.

Pogrebnyak Shots

Pog Shots

Ben Basat Shots

Ben Basat Shots

Pretty much all of Pogrebnyak’s shots were struck towards the central portion of the goal, whereas Ben Basat’s shot placements seem almost remarkable.  The central portion of the goal, the area that leads to easy saves for the goalkeepers has a noticeable absence of footballs – there were simply no shots aimed there.

eden-ben-basat
Either Ben Basat was lucky in where the shots were targeted or else the Israeli has a serious football brain on him.  A clear pattern can even be seen in the shots that he struck off target, nearly all of them are arrowed towards the bottom corners.

Of course, by attempting to seek out the corners of the net he is going to have his share of off target shots, but with those shots which do hit the target attracting a success rate of five times that of the centrally struck shots so favoured by Pogrebnyak I know which player is likely to turn more of his shots into goals.

Parting Thoughts

In this piece I have set out why I feel that the traditional shots on target metric has its shortfalls in assessing player accuracy, I’m just not sure it’s measuring the correct thing.
Perhaps in a future piece I will have a look at creating a metric that I feel better reflects the expected goals value of shots that a player took, which after all  is what the shooting accuracy metric is supposed to do in my opinion.

Article by Colin Trainor