Sunday, 19 May 2013

Possession, Opponents And Match Outcome.

One of the early additions to the usually quoted football stats of goals scored and conceded was the amount of possession enjoyed by each side during a match and as such it is almost universally quoted today. It is therefore understandable that much effort has gone into determining the connection between possession and success or otherwise on the field.

Naturally from a supporter's viewpoint it feels more secure if your own side has the ball and similarly the ability to keep possession is often, quite rightly connected with talent and skill. If we further include that the most successful and well resourced teams in most major leagues are largely based around a passing and therefore possession based style it is easy to see how increased amounts of possession became intertwined with an increasing likelihood of achieving a favourable outcome.

However, it is increasingly becoming apparent that the relationship between possession and wins is far from straightforward. In this post  I outlined an earlier view that possession merely tells you how long teams spent trying to do certain things on the pitch. It doesn't tell you what those actions were, it doesn't tell you how successfully the actions were translated into really important things, such as goals  and it doesn't tell you how effectively each side carried out those tasks. Game states, tactical approaches and relative skill levels between the sides  and of course randomness, decide match outcomes and how these are played out on the day decide the largely secondary statistical measure that is recorded as possession.

Extremes can often be used to illustrate more subtle differences which appear in all matches but are difficult to spot when the teams play with similar natures and intent. Barca are unlikely to ever compete in everyone's dream matchup against Stoke at the perennially wet and windy Britannia, but pass loving Arsenal provide an adequate proxy for the Catalan giants. The outcome is fairly predictable at the Emirates, but less so at Stoke, where the Potters often claim all three points. But one universal constant persists, namely win, lose or draw, Arsenal always have much, much more of the ball than Stoke.

The match outcome is decided by the interplay of Stoke's direct, set piece centric approach, where defending is a chore undertaken largely without the ball, pitched against Arsenal's weaving, intricate brand of passing. Possession stats merely fall into place at the end of the game as a by product. So if possession in the case of Arsenal and Stoke contests is a predictable variable based around team styles, that his partially hard baked into their contests and is largely independent of match outcome, how do other less stylistically extreme matches fare?

Below I've plotted the amount of actual possession enjoyed by Stoke and Arsenal in every game from 2011/12 against the average possession of their opponents over a representative selection of matches.

The trend is clear and even more prominent across other EPL teams. A side's share of match possession is tied to the historical tendencies of both itself and it's opponent. For example , when Stoke meet a side which also employs a tactical approach that shuns possession, then Stoke's share rises and when Arsenal face similarly possession loving sides their share falls. In short, the possession battle is decided, largely before a ball is kicked and while it can be shifted slightly by such things as red cards, venue and scoreline, it is partly predictable with reference to the past styles of the competing teams.

The Stoke plot particularly highlights the futility of trying to connect possession stats to match outcome without reference to a side's preferred, and presumably most effective playing style. Stoke won eleven games in 2011/12, all achieved with less than 50% possession and they dominated possession in three games, winning none of the three.

League wide the connection persists. Pregame, historical possession stats for both sides, along with venue can predict individual match possession relatively well, as demonstrated by the plot below. Red cards in particular produce distorted extremes, but prior knowledge of each side's possession history leads to an adequate estimation of how often each side will see the ball in a single match.

Stoke have managed to secure virtually all of their Premiership wins with less than 50% of match possession, and to sever the last remaining connection between possession and performance, we need to see if more possession relative to a side's normal, average share, as opposed to the lion's share of possession, correlates to more successful results.

Spurs were in a more celebratory mood when seeing less possession in 2011/12.
 Defining performance over a single match is difficult because of the discrete nature of 3, 1 or 0 points awarded for each possible outcome, but we can partly overcome this by seeing if above average performance compared to pregame estimates are seen where possession figures are also above average for individual teams.

Stoke won 85% of their pregame points expectation combined in matches when they had above average( for them) possession, but 125% of their pregame points expectation when possession fell below their typical average. So more possession was generally an indication that Stoke were doing time consuming actions that, for them, were connected to under performance. Other teams shared this trait, from Everton, Spurs and QPR to both Manchester clubs.

Team's spread the length and breadth of the EPL table from Arsenal, Chelsea to Bolton, WBA and Villa demonstrated the reverse preference. Possession figures above their average led to better than average results and the less they saw of the ball the poorer their relative performance became.

Once again we are looking at what teams did and how long those actions took and ultimately how successful and lucky they were with their changing approaches. Possession largely appears to be a statistic that more defines a side's stylistic approach to gaining, defending, retrieving a desired result. As a stand alone number, it's usefulness fails to survive the inevitable lack of more detailed context driven investigation.

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