Thursday, 16 January 2014

Stripping the Luck from Shots from Outside the Box.

Following on from the last post on conversion rates from inside the box, with penalties and headers removed, here's the same analysis for shots from outside the box.

Any set of repeated trials where there is only two possible outcomes, a success or a failure, will over show a random variation over a series of team repetitions, even if every team has the same true talent and each trial is identical. Toss lots of fair coins, grouped as teams and some will appear talented and some will appear below average.

If there is variation in the levels of talent, the spread of the recorded conversion rates is going to be wider than you would expect from a group of results produced by equally talented sides. In the case of a side's ability to convert shots from the outside of the box (quite a large area compared to other competitive team sports), we can hope that the opportunities presented to each side even out with increasing sample size and any deviation from the expected results will be down to different, repeatable levels of team skill.

In short, we are trying to see the amount of variation in conversion rates that is down to team talent, once the ever present random component is removed.

We aren't saying that scoring from distance is entirely luck, because it clearly requires great skill. But we are trying to see, using the available data, if the difference in observed conversion rates for EPL sides is down entirely to luck. In other words, all teams are highly skilled and equally talented and the observed differences are just down to random variation. Alternatively, the spread of the conversion rates across the league, might imply that there is also some degree of true talent differential. In other words, we are looking at the best of the best, but some are slightly better than others.

In the previous post on shots from inside the box, the "extra" deviation that implied a repeatable talent was possibly present, diminished once we removed headers from the sample, because the proportion of headers that make up all attempts within the area greatly varied for sides in the 2011/12 season. Shots from outside the box are almost exclusively from kicks, therefore, this "cleaning up process" by removing headers isn't possible. However, shots directly from free kicks are the closest, atypical group of attempts from outside the box that can be culled from the larger sample to improve the consistency of the trials under investigation.

Firstly, dead-ball shots carry a greater goal threat than open play efforts, (a dead ball shot from outside the box is roughly the equivalent of a shot from open play, but half a dozen yards closer to goal). This is partly because the kicker can compose himself before the effort, but also because a side's most talent striker of a ball can be used, rather than a wider selection of players in normal, open play. if you include direct shots from free kicks, there is potential to increasingly distort both the distribution of attempt quality nd the make up of the players attempting the shots.

So in this analysis, all open play shots (with the feet) from 2011/12 from outside the box, for each of the 20 EPL sides are used and the strike rates for all 20 teams is recorded. The distribution of this actual 20 team strike rate is then compared to the expected distribution that is likely if each side was identically talented at shooting and converting from distance, allowing for the actual number of attempts by each side in the 2011/12.

Unlike foot shots from inside the box from the previous post, the range of conversion rates does appear much greater than the expected range from a random, equally talented draw. In short, there appears to be a larger talent component than that seen in shots from inside the penalty area, in the EPL during the 211/12 season. (Or we have a widely different range of the quality of the opportunities trialed by each side).

Similarly, we don't know if the most efficient converter of shots from outside the box were an exceptionally talented side that got unlucky, a good side that got marginally lucky or a mediocre side that got incredibly lucky. But, over the range of all 20 sides, we can regress the actually recorded conversion rates based on the amount of random variation that appears to be present, in an effort to improve the validity of the numbers.

Removing the Random Variation from Conversion Rates from Outside the Box.  

EPL Side 2011/12. Actual Conversion Rate. % Regressed Conversion Rate. %
Manchester City. 7.3 6.1
Manchester United. 5.8 5.0
Wigan. 5.1 4.6
Aston Villa. 4.4 4.4
Tottenham. 4.3 4.2
Arsenal. 4.0 4.0
Sunderland. 3.6 3.8
Everton. 3.6 3.6
Swansea. 3.5 3.5
WBA. 3.5 3.5
Newcastle. 3.4 3.5
Bolton. 3.0 3.2
Blackburn. 2.7 3.0
Stoke. 1.6 2.5
Liverpool. 2.0 2.5
Norwich. 1.9 2.5
Wolves. 1.7 2.4
Fulham. 1.8 2.4
Chelsea. 1.7 2.2
QPR. 1.6 2.1

The analysis is far from idea. Sample size would ideally extend beyond one season and quality of opportunity must still remain a strong candidate for the extended range of conversion rates. However, quantity of opportunity is considered and, as with shots from inside the box, the top four sides are to be found in the top six of converting sides.

No comments:

Post a Comment