Thursday, 4 July 2013

Efficiency Stats and Team Talent .

It is relatively easy to obtain quite detailed team or individual statistics for most of the major European leagues, most notably the Premiership. Sources and interpretations do vary, therefore it is important that consistency of source is ensured before conclusions are made. The new data is at it's most useful when presented in the form of efficiency figures. The margins within the Premiership are often extremely tight and a team that can perform a task a couple of percentage points better than their rivals may have a decisive edge in fulfilling their aims.

Team style and tactics of course vary greatly between teams in the same division and so looking at how effective side A is when hitting the target with their goal attempts compared to team B is likely to be because of a clash of styles rather than a raw talent differential. Side A may merely engineer shooting or heading opportunities from closer to the goal or in markedly different proportions to side B. Therefore if we look at such figures across all teams, inevitably a difference will be apparent, but often the resolution will require detailed knowledge of the approach favoured by each side.

A much simpler method is to look at efficiency stats, such as the ability of one single side to hit the target, spread over multiple seasons to see if there is a significant, attributable change in this rate over time.

The rate at which a side performs tasks such as getting their shots to hit the desired target will of course fluctuate over multiple seasons through random variation, even if there is no change in the side's inherent talent levels. However, if the seasonal rates change to a larger extent than should be expected due to simply chance fluctuations we can begin to suspect that something or someone within the side has engineered this change. Looking for possibly significant changes in a largely stable team format over a limited number of seasons, should be possible.

I've used simple "success/fail" shooting data for the twelve ever present Premiership teams over the last five seasons, so the teams involved are as follows. Manchester City, United, Everton, Liverpool, Spurs, Arsenal, Chelsea, Wigan, Sunderland, Fulham, Villa and Stoke. All data looks at each side from an attacking perspective, because, unfortunately defensive shooting numbers are much less easy to come by or collect.

Attempts On Target.

Half of the sides appeared to show seasonal fluctuations that weren't wholly consistent with the usual random fluctuation in their ability to find the target with all of their goal attempts given their average success rate over the five seasons. The biggest likelihood that we are seeing a real effect came at Manchester United, where the change in accuracy was accompanied by a near relentless improvement from a likely true accuracy of just under 32% in 2008/09 to 37% in 2012/13. The first major improvement came in 2010/11, this was repeated in 2011/12 and consolidated at peak levels in 2012/13.

Sunderland appeared to show the next highest levels of fluctuation, recording five year highs in 2009/10 of 34.5%, but immediately following  up with lows of 28% the next season. Fulham showed and maintained distinct improvement in shooting accuracy from the start of 2011/12.

Teams Showing Change. (Most Likely to Least) Season High Accuracy Rate. Season Low Accuracy Rate.
Manchester United. 37.0% in 2012/13. 32.4% in 2009/10.
Sunderland. 34.6% in 2009/10. 28.6% in 2010/11.
Fulham. 35.2% in 2011/12. 30.2% in 2008/09.
Wigan. 32.9% in 2012/13. 29.1% in 2008/09.
Liverpool. 33.2% in 2010/11. 30.4% in 2009/10.
Spurs. 33.7% in 2012/13. 31.7% in 2010/11.

All of the remaining six sides exhibited shooting accuracy figures over the five seasons that did vary, but not by enough for random chance alone to be ruled out as the sole cause.

Converting On Target Attempts.

If we now move on to conversion rates for on target shots, remembering that we are not looking for the most or least efficient teams, but for a side that has shown greater than expected efficiency fluctuations over a five year period and it may be possible to attempt to isolate some of the potential causes for the observed change.

Only three teams showed sufficient variation to raise suspicion that on field actions or a different squad make up had lead to real, repeatable and controllable change over seasons and they were Liverpool, Chelsea and Manchester City. The best evidence from the raw conversion figures came at Anfield, but there was similarly strong output from the other two sides. The most significant conclusion, here is that the vast majority of sides failed to demonstrate an ability to increase or a failing to decrease their goal expectancy from the shots they did manage to get on target.

Teams Showing Change. (Most Likely to Least) Season High Conversion Rate. Season Low Conversion Rate.
Liverpool. 31.5% in 2008/09. 22.3% in 2011/12.
Manchester City. 35.3% in 2011/12. 29.6% in 2012/13.
Chelsea. 33.0% in 2009/10. 27.6% in 2008/09.

Having Attempts Blocked.

In the previous two categories higher percentage rates are desirable, but it is more ambiguous here. Seeing a shot blocked tends to be bad news overall, although if your opponent chooses to block a shot that is going wide at least the ball remains in play and available to all. The likelihood of an effort being blocked is often a function of shot type, shot distance and therefore, also game state. So there are more likely to be causes that may not be directly attributable to mere team approach in this case.

Seven of the sides appeared to be able to alter their opponents blocking ability outside of random variation over the five year period. Again there were no borderline cases, so in these seven cases the fluctuations may  have a cause if you are prepared to look hard enough. In the table below, again remember that a high rate of blocking is probably a bad thing for the team trying to score.

Teams Showing Change. (Most Likely to Least) Season High Attempt Blocked Rate. Season Low Attempt Blocked Rate.
Liverpool. 28.1% in 2009/10 and 2012/13. 22.8% in 2008/09.
Chelsea. 28.6% in 2008/09. 24.1% in 2011/12.
Manchester United.
29.7% in 2008/09.
23.5% in 2012/13.
29.8% in 2010/11.
23.2% in 2008/09.
Manchester City. 29.2% in 2010/11. 24.1% in 2008/09.
Aston Villa.
27.0% in 2012/13.
23.1% in 2008/09.
Arsenal. 27.8% in 2012/13. 24.6% in 2010/11.

Converting Clear Cut Chances.

Villa took good advantage of clear chances in 2012/13, although this one got away.
We are down to just three years of completed seasonal data in this final category. However, the more restricted definition of a clear cut opportunity may make the smaller sample size similarly valuable because of the more consistent repeatability of each goalscoring "trial". There is very strong evidence that Manchester City's ability to convert so called clear chances has changed even over three season, along with much less convincing clues for Sunderland and Villa, with Arsenal and Liverpool sandwiched in between.

Teams Showing Change. (Most Likely to Least) Season High Clear Chance Conversion Rate. Season Low Clear Chance Conversion Rate.
Manchester City. 42.0% in 2011/12. 30.0% in 2012/13.
Liverpool. 42.3% in 2010/11. 31.5% in 2011/12.
Arsenal. 39.6% in 2012/13. 31.3% in 2010/11.
Aston Villa. 38.9% in 2012/13. 31.5% in 2010/11.
Sunderland. 35.9% in 2012/13. 29.5% in 2011/12.

This post is as much about the sides that don't appear in any of the above tables as it is about those that do. Stoke's shooting accuracy ranged from 25% in 2011/12 to 29.5% the previous year, but given their league low shooting attempts, combined with data from the other three seasons, those figures are perfectly consistent with being a product of their average rate over the five season period of 28%. You may be embarking upon a fool's errand if you try too hard to rationalize the drop from the Potters from 2010/11 to 2011/12.

This post has been about where and when to look and where not to look for substantial change in teams and it also highlights that an apparent drop in efficient output may be simply down to random chance with little change in team ability. A side my appear to falter in a recorded stat, but underlying talent remains if it is given more time to show itself.

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