Monday 13 August 2012

Defence. It's All About The Marking.

Imagine a goal scoring opportunity 12 yards from goal and dead centre, in other words on the penalty spot. What's the likelihood that the attempt will be converted ? Naturally the answer will depend on a variety of factors. An actual penalty will carry a greater chance of being converted than will a header from a corner in a congested area. In the former situation defenders are totally absent and the striker has an unimpeded shot at goal, success rates will be in excess of 70%. In the latter the presence of a large number of defenders, with one probably assigned marking duties on the chance taker will make converting the opportunity much more difficult. Shooting is likely to be more hurried, less accurate and potent, there's also the added possibility that the subsequent attempt will be blocked before it even reaches the keeper and success rates will drop considerably towards 1 in 20 levels.

In previous posts I've looked at how the position on the pitch from where a goal attempt originates has a significant bearing on the likely success of that attempt. Wider and further quickly reduces conversion rates and headers quickly lose out to shots as we retreat from the six yard box. However, we haven't yet attempted to incorporated the impact of defensive bodies around the striker and between the ball and the goal.

Collecting information detailing the amount of bodies in close proximity to a goal scoring attempt is a hugely labour intensive exercise requiring the use of video analysis. However, we can use a reasonable proxy by looking at the situation from which a chance originates. Set plays such as corners and free kicks where the ball is played into the box rather than being fired directly at goal are likely to see players most tightly marked. Chances created from open play will afford attacking players the opportunity to find more space and separation because of the more fluid nature of the move and a much less compressed area of play compared to a corner. Finally, counter attacks where opponents have committed players to the attacking third of the field gives the countering team the best opening to provide a final pass to a colleague enjoying time and space to shoot at goal.

Corner Kick.........All Link Arms.
To measure the impact that tight or loose marking has on the conversion rates of shots taken from various distances and angles, I've used the average overall goal expectancy derived from the full data set and compared the predicted figures to goals scored from actual attempts made in each of the three categories.

Scoring Rates In Various Defensive Situations Compared To Overall Scoring Rate.

Likely Defensive Formation. Actual Goals/Expected Goals.
Tight. 0.65
Normal. 0.96
Stretched. 2.12

Chances created from open play that are likely to be characterized by "normal" levels of marking predominate in the data, so the conversion rates for "tight" and "stretched" defences shouldn't be taken at face value, but the results appear to be as expected.

Close, man to man marking prevalent at corner and free kick situations depresses the rate of scoring to only 65% of the overall conversion rates, but stretched defences subject to quick counter attacks positively leak scores at over double the usual rate.

Average Scoring Probabilities Against Various Defensive Formations Sorted By Attempt Distance from The Byeline.

An even starker example of the importance of an organised and fully manned defence is seen if we run a regression on the goal concession rates of our three individual types of defences and plot the average predicted conversion rates for attempts made throughout the penalty area at varying distances from the dead ball line.

Undermanned and possibly disorganised defences on the receiving end of counter attacks are overwhelmingly more likely to give up a goal than are "normal" or "tight" defences. Sample sizes are again small for attempts made on the counter, but the likelihood of each type of defence conceding goals only begins to converge when shots are taken from around the edge of the box. At all other distances within the area, stretched defences are much more fallible than normal ones, who in turn trail defences set up to defend corners and free kicks. In my dataset, an opportunity created inside the six yard box from a counter attack is over twice as likely to succeed as a similar chance created in normal open play or from a set piece.

The difficulty of scoring from corners and set pieces where the ball is played into the box is again illustrated using this approach. There is clear indication that corners are relatively unproductive, not because of the angle of delivery, but because of the time allowed for the defence to organise and populate itself. Defenders work extremely hard, both inside and outside the laws to stay close to attackers at such set pieces and with good reason. Similarly attackers who can free themselves, perhaps with the help of a blocking run by a colleague can reap rich rewards by moving the chance into an environment more akin to a counter attack.

It should come as no surprise to discover that a well organised defence can degrade the potency of an attack, but the extent to which this appears to happen may do. At the very least this kind of analysis further highlights the misleading nature of possession statistics. Teams that are content to soak up pressure and occasionally hit opponents quickly on the break are likely to create fewer, but better quality opportunities.

Labeling teams that dominate possession and chances as undeserved losers may soon be a thing of the past as the narrative is replaced by one where well organised teams forego possession to create premium opportunities on the counter and start to be described as the deserved winners. The more we delve into the statistics, the more apparent it becomes that there is more than one way to win a football match.

1 comment:

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