Wednesday 18 July 2012

Getting Out Shot Isn't Always Bad News.

With the influx of readily available data, shooting efficiency has quickly become the mainstay of much of the statistical analysis of both football teams and individual players. The rate at which a keeper keeps out shots, the ratio of goals to shots for strikers and team conversion rates are now a common and welcome addition for debate. While any advance in the quantity of data is to be welcomed, this all encompassing approach does have it's drawbacks. In judging a keeper or a striker simply on his shooting or saving success ratio we are hoping that sheer weight of trials are leveling out the quality of the attempts. The accuracy of our conclusions depend on keepers facing roughly the same overall quality of attempts or players or teams being presented with chances of roughly the same difficulty. An improvement compared to the previous goalcentric models perhaps, but still a not inconsiderable leap of faith.

Euro 2012 well demonstrated that there are numerous ways to play football and teams as diverse as Stoke and Swansea can achieve virtually identical seasonal points tallies in the Premier League by employing polar opposite approaches. By using raw counting methods and applying it to such statistics as team shots, we may capture the general trend, but we are destined to miss almost all of the finer detail that may differentiate between the Midland's best EPL team of recent times and their Welsh cousin.

Therefore a parallel approach is needed using more granular data or we risk dismissing alternative playing styles as mere outliers or the product of excessive luck. Everyone is comfortable heaping praise on the all conquering Spanish club or country sides, who are perceived as maximizing their talent with a style and philosophy that is both successful and pleasing to the eye. But we still need to try to explain why less attractive teams appear to prosper, especially if their success appears to contradict the new statistics that are currently used to define desirable team talent.

The first step to improving the information gained from shot data is to be able to attribute a expectancy for each shot or header based on the origin of the attempt. How likely is a shot to result in a save, a miss, a block or a goal. This tentative first step is available from Opta supported apps, such as FourFourTwo's Stats Zone and while data collection is time consuming, the effort is rewarding.

The continued comfortably successful presence of Stoke in the EPL midtable and within the various domestic and European cup competitions regularly provokes reactions ranging from mild bemusement to outright hostility. Especially as they are entering their fifth consecutive top flight campaign while appearing to perform poorly in many of the shot based advanced metrics and possession stats.I'm extremely skeptical about the usefulness of possession data ,but for now I'll use Stoke to illustrate how shot data can reveal hiddens gems about a team's record, but only if we look beyond the general counting stats that are the mainstay.

I've taken every goal attempt propelled and faced by Stoke during the 2010/11 season, their third year back in the top flight. The declining influence Delap's longthrow has been charted here and the goal scoring duties for the campaign lay at the feet and head of £8 million Kenwyne Jones, the largely unheralded £3 million Jon Walters from Championship side, Ipswich, Tuncay, a £5 million flop who would leave in the January window and an ageing and increasingly injury prone Ricardo Fuller. So the striking talent was broadly consistent with that available to EPL teams from midtable and below.

How Many Goals A Slightly Above Average Side Would Expect to Score Given Stoke's Opportunities in 2010/11.

Stoke Attack (480) Goals. Shots on Target. Shots Blocked.
Expectancy. 69 183 117
Actual Performance. 45 135 128
Expectancy per Shot. 0.15 0.38 0.24

My shot conversion model at the moment contains an over representation of good attacking sides, but we are simply using it here to see how Stoke managed to overcome their unbalanced ratio of shots allowed and shots attempted.

Stoke's tactical approach under Pulis has always been one of containment regardless of venue, so the lack of possession is baked into the gameplan. They were outshot in 2010/11 by almost 60 attempts over the season as a whole, but finished 13th with a goal difference of minus two, an extra point away from 10th. The point of origin of each Stoke attempt has been charted and a goal expectancy, accuracy expectation and likelihood of seeing the shot blocked is calculated for each shot. Cumulatively we can then see how many goals an above average attacking team might have scored had they been able to create Stoke's 480 chances from 2010/11.

How Many Goals A Slightly Above Average Side Would Expect to Concede Given Stoke's Opponents Opportunities in 2010/11.

Stoke Defence (539) Goals. Shots on Target. Shots Blocked.
Expectancy. 59 186 142
Actual Performance. 46 182 156
Expectancy per Shot. 0.11 0.35 0.26

The predicted outcomes of the shots Stoke's defence allowed are then similarly treated and by comparing the two batches off shots we can try to see how Stoke managed to score as many as they conceded despite the large excess of shots aimed at their goal by frustrated opponents.

The most significant figure is the expectancy per shot in each table. When Stoke were shooting it averaged 0.15 per attempt, when they were on the receiving end the figure fell to 0.11. So Stoke were creating less shooting or heading opportunities and allowing more, but this was compensated by their shots on average being more likely to result in goals. They were also more likely to be on target, creating rebound possibilities and less likely to be blocked judged on where on the pitch the attempts were taken from.

That Stoke's strikeforce didn't manage to match the goal tally from their 480 attempts that was predicted by a model that contains data from the likes of the top five EPL teams shouldn't be surprising. But Stoke scored 45 goals, omitting own goals in 2010/11 by presenting relatively low numbers of high quality chances to predominantly average or below Premiership strikers. Leading to a average season long total in the mid forties. As a group Stoke players who had a goal attempt two years ago weren't that potent, certainly weren't very accurate and had more shots blocked than the top teams......but they were presented with top drawer chances.

Kenwyne..."...set them up & I'll put some of them away".

An EPL side consists of at least two units and the defence, by contrast overperforms the model. Even though they had an average of two more shots a game to contend with compared to their own team's offensive output, they only conceded 46 goals. Chances created by the opposition were more difficult opportunities, as shown by an expectancy per shot of 0.11 goals compared to 0.15 goals for the Stoke attack. Given Stoke's setup, that was probably a team effort, starting with Jon Walters and working back down the field. Opponents also couldn't match the model's expectation and under shot by 13 goals and that was probably more down to the back four and keeper. Stoke's defensive legion also made many more blocks than predicted, another constant from team's coached by Tony Pulis.

So in this case the more numerous shots allowed were of a lower chance quality and intervention by Stoke defenders, which naturally is currently beyond the scope of one man analysis, further reduced the scoring levels.Stoke's 480:539 shot ratio appears poor, but the raw numbers obscure the hard work of the defence and a gameplan designed to produce quality chances for a team then lacking in attacking excellence.

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