Friday 26 July 2013

Unpeeling The Shooting Onion.

The quality of a foot shot in football is decided by a variety of parameters. This list, not necessarily an exhaustive one, includes shot location, the power of the attempt, the placement of the shot and the defensive pressure on the shooter. Most of these listed can, at least in some general way be recorded and quantified to allow further analysis. However, other possibly significant factors, such as pitch condition, player fatigue and quality of the previous pass are less easy to tabulate.

It is easy to overestimate the amount of control a player has over the execution of a shot. Variables, such as defensive pressure present an obvious and changing irritant for the shooter, but constants within the context of a single shot, such as shot distance may also apply some influence on the type and hence the quality of the attempt.

For example, distance shooting often requires a more powerfully struck shot, not only to make blocks by outfielders less likely, but also to get the ball to it's target quick enough to prevent the keeper from correctly positioning himself. But the requirement for greater power may relieve the player of some control of other aspects of the shot, such as placement.

Given sufficient time a keeper's movement and positioning allows him or her to cover the entire goal. But shots taken closer to goal may be hit with less power, but arrive quick enough at an area of the goal that the keeper is unable to reach in the time available. Therefore, distance shooting may require power to hurry them on their way and closer efforts may rely more on guile and craft to consistently direct the ball to it's intended target.

I've plotted the correlations between these more commonly charted shot characteristics and various shot outcomes. We may then try to build up clearer picture of how they are likely to relate to outcomes at the sharp end of the effort and to allow speculation as to whether the shooter is controlling the relationship or if the initial inputs are at least partly hard baking the outcome.

Shot strength is broadly divided into powerfully, normally and weakly hit shots and the three types show a gradual, significant trend as we get closer to goal. The perpendicular distance from the goal in yards is used to denote the x,y coordinate of each attempt.

Most shots are hit with normal power (obviously, from the name alone), but the percentage increases as we near the goal. The decline as we enter the box is keenly felt in the proportion of strongly hit shots. Over a quarter of shots are given a real thump from 30 yards, but less than 5% from the edge of the six yards box. Are we seeing the need to get the ball quicker to the target being proportionally more important from distance being displayed in the data ? Closer in, placement appears to become more important and presumably profitable than raw power.

The  interesting category is weakly hit shots, surely never an advantage for the ball striker. The steady increase in the proportion of weak shots as we approach the goal is possibly due to the higher intensity of defensive pressure or the eagerness to get a shot away from close range regardless of quality. Miscues will always happen, but something appears to be encouraging them in the six yard box.

The next graph is essentially a "high or low" plot for shot placement dependent on the strength of the shot. Shots from distance that are on target tend to be more likely to hit the goal plane vertically above the half way point. Possibly another manifestation of distance shooting being more powerful and tending to go high compared to placement orientated closer efforts.

If we imagine a goal kick as an extreme variation of the long distance shot, it is hit with power (to prevent opponents blocking the effort), it is hit aerially (again to reduce blocking, but also to carry the required distance, maximum range is achieved with a trajectory of 45 degrees to the horizontal). By contrast a pass or a short goal kick is hit with just enough power and more placement to avoid an opponent, but safely reach it's destination.

Long distance shots maybe the goalkicks of the shooting world and shorter ranged shots are the passes

We can further refine the shooting tendencies by looking at the preferred destination for on target efforts in regards to corners of the net, either top or bottom, again sorted by shot distance. Shots from distance are nearly twice as likely to arrow towards the top corner than the bottom. A controlled effort by the striker to try to hit the most difficult area for the keeper to reach or merely the natural outcome of the relative inability to keep powerfully struck, long range efforts from flying to an elevated height ?

The final graph looks at the sacrifice in accuracy you give up by hitting a more powerful shot compared to a normal one. Powerful shots are more likely to miss or be blocked than weaker, or normally hit efforts.

To summarize, from distance (30 yards), powerfully struck shots are at their peak, they take the aerial route more often, hit the top rather than the bottom corners of the goal, but lack the "on target" accuracy of normally struck efforts. A player can certainly chose how hard he hits a shot, but that the attempts then more frequently take to the air may be at least partly down to the difficulty in controlling the flight of a powerful shot, rather than conscious aim taking.

Once we home in on the penalty box, the areas of the goal that a keeper cannot cover adequately in the timescale of a normal shot rapidly increases. Hitting top corners or going for power is no longer a priority if "passing" the ball into an area of the goal that a keeper's angles have left exposed carries a higher likelihood of success.

Shooting appears to be a constant interplay of trade offs between power, placement and a multitude of other variables, with distance partly dictating the required approach and then possibly having a say in the most likely end point of the effort. It is easier to manufacture a shot from distance, but the skillset and rewards may differ from those needed from closer to goal and it is easy to speculate that the spread of talent for players taking shots may well differ at different distances from goal. Are there more Bale's than goal poaching Lineker's in the general striking population.

The introduction of more variables certainly widen the scope for evaluating shooters. Ultimately though, at the moment, we don't even really know how much of the process is fully in control of the feet of the shooter.


  1. mark - great post. re weakly hit shots, many of these would likely be chips. i've not seen any studies looking at/sorting out chips from other shot types but it seems like you have the data to get at that question.

  2. Hi anon,
    I know that some clubs are using shot trajectory as an input in their shooting models, but unfortunately I don't have enough of that type of data to include it at the moment.

    You make a great point, a weakly or even normally hit shot that rolls along the ground and is picked up in the centre of the goal is very unlikely to result in a goal. But a chip that in the absence of trajectory data could appear identical to the previous example would carry a much higher likelihood of a goal.

    I'd imagine the former greatly outweighs the latter, judging by the average conversion rates for x,y positions where a chip would have been a viable option.