Thursday 3 December 2015

Diego Costa, Head & Shoulders Above the Rest?

There have been some great stats on potential finishing ability posted here on Dan Kennett's twitter feed. Naturally the focus was on Liverpool players, particularly Daniel Sturridge and the post proved timely following Wednesday night's 6-1 away victory at Southampton.

Identifying different levels of finishing ability is always going to be challenging in a sport where scoring opportunities are relatively rare.

Squad rotation, substitution and injuries often deprives strikers of playing time and few manage more than five attempts per 90 minutes.

Even in Dan's comprehensive list of the highest achievers, only a handful of players have exceeded 10,000 minutes since 2011, the time it allegedly takes to master a skill.

Topping the list of currently active Premier League strikers is the recently rested Diego Costa. A 23% conversion rate has been achieved in less than 100 attempts, a small sample size compared to the remaining players in Dan's list, who average nearly 300 attempts each.

Therefore, although Costa's conversion rate is well in excess of Aguero and Sturridge, his nearest EPL challengers, there must be a suspicion that his 23% rate is unsustainable, long term compared just under 15% for the other two.

Who Needs a Fit 16% Striker Every Week!
There are a variety of approaches that are currently available try to identify finishing skill.

Expected goal models add an extra level of insight. But they are data hungry and potentially susceptible to rare events, such as deflected shots. Ultimately they only measure a player's deviation from the norm expected by that particular shot based model, which itself is almost certainly incomplete.

It is also rare to see such model based analysis address the likelihood that any over or under performance occurred merely by chance.

If instead we assume the chances presented to these out and out strikers are broadly similar, we can see if the spread of conversion rates is wide enough to imply differing levels of finishing skill within the chosen group.

This approach focuses more on the role of random chance and incorporates sample size, while assuming chance quality is similar for each player.

In short, it is a flawed, polar opposite approach to that of an equally flawed shot based model.

Regressed Conversion Rate for EPL Strikers 20011/12-2015/16.

Data Credit - Dan Kennett.

Player Regressed Conversion Rate.
Costa 0.149
Hernandez 0.148
van Persie 0.147
Aguero 0.147
Sturridge 0.147
.............................. ..........................
Defoe 0.142
Lukaku 0.141
Suarez 0.141
Ba 0.139
Bony 0.139

The spread seen in Dan's numbers are just extreme enough to conclude that there is some evidence that finishing skill may exist. 

Costa remains the highest rated finisher, but his numbers are regressed by over 90% towards the group average because of his relatively low number of attempts. We have to go to the third decimal place to elevate him above the next four highest players, including Sturridge.

Similarly, the gap between the most and least efficient finisher is now just 1%, rather than the 12% seen in the raw data.

It would be unusual to see a wide range of true finishing abilities at the elite level of a professional sport. 

There may be tentative evidence to suggest that a narrow gap does exist (perhaps traditional scouting could contribute the eye test) and Daniel Sturridge is towards the top of such a pecking order...but can he do it on a bitterly cold night in a minor cup competition in January at the Britannia Stadium!

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