Saturday 23 June 2012

Shooting Efficiency.Covering All The Angles.

Knowing the kind of numbers that a league average team is likely to produce over a range of in game performance indicators is an almost essential tool in analysing a football team.For example the average number of goals per EPL game has hovered around 2.5 over recent years and this number is split about 1.4 goals for the home side and 1.1 for the visitors.Therefore,if a team has scored an average of 2 goals per home game over a representative number of match,say a whole season,then we can be fairly confident that they possess an above average attack.Knowledge of the average immediately places an individual team either above or below that mean mark in the pecking order.

Unfortunately,while this kind of data is easy to collect for the more common place measurements such as goals,once we move towards even common advanced stats such as shot conversion,we begin to encounter limit and incomplete sets.Therefore in trying to produce an average conversion rate for shots spread over the length and breadth of the pitch,I am not going to pretend that the following analysis is definitive.It is at best ballpark and biased towards the kind of teams for which I have collected chalkboard shot data.

Unsurprisingly,we've seen in various posts that distance from goal is one of the main defining factors in how likely it is that a shot or header will result in a goal or even if a shot ends up on target or not.The further the ball has to travel to reach the goal,the less potent and less accurate those attempts become.However,I'd now like to go a step further and try to incorporate the actual position on the pitch from which the attempt originated,thus incorporating the effect shooting angle has on conversion efficiency.In addition I'll try to show which shots are more likely to be blocked,giving a route to evaluate the effectiveness or otherwise of individual team defensive actions.

Most shots originate from the central area area of the pitch at varying distances from the goal.So I've mapped each goal attempt in this analysis using the centre point of the goalline (roughly where the keeper stands to face a penalty) as the origin to record the width wise position of the shot and the perpendicular distance to the deadball line to signify the length wise co ordinate.Therefore,for example a penalty shot would have a  width co ordinate of zero yards and a length co ordinate of 12 yards.

I've regressed the outcome of every shot in my data set (around 4,000 attempts) against the co ordinates for the shot's origin to obtain the likelihood that the shot will be either blocked,on target or result in a goal.This allows a methodology to produce an average goal or shot on target expectancy to be calculated for any shot made in the EPL from any position on the field.With access to much larger datasets ultimately this will allow comparisons to be made between league average values for shot accuracy and conversion and those of individual teams or players.It can also allow us to evaluate how unlikely a player is to score certain goals and at the end of this post I'll try to put a numerical value on the two Goal of the Season contenders from 2011/12.

Rather than present a regression equation that is almost certainly biased towards my sample of teams,I'll graphically represent how the likelihood of success begins to fall away as attackers are forced to shoot from wider and wider angles from distances ranging from 12 yards out (level with the penalty spot),18 yards and finally 30 yards remote.I've compared the lateral distance effect of a central shot (in line with the penalty spot),one 4 yards either to the left or right of centre (in line with one of the uprights) and finally shots that are taken from 7 yards wide of the centre of the goal.

How the Likelihood of Scoring Decreases as Shooters are Forced Wide.

Modelling from the available data,the benefit of forcing an attacker laterally across the pitch prior to a shot is well demonstrated.A shot from around the penalty spot is going to result in a goal about once in every four attempts,but the conversion rate halves if a player has to shot from level with the spot but seven yards wider.The changing angle and potentially longer distance that the shot needs to travel to reach the goalline would appear to contribute to reduce the goal expectancy of such attempts.The effect is present at all distances.24 shots on average are required per goal from 30 yards out and central in position,but this leaps to over 50 if the attacker is pushed wide of a perpendicular line to the uprights.Shots unwisely taken from 30 yards out and level with the perpendicular of the edge of the area succeed once every 300 tries.

How the Likelihood of Shots Being On Target Decreases as Shooters are Forced Wide.

For shots on target I've included attempts made from as wide as the perpendicular of the penalty box because it's inclusion still maintains informative height proportions in the bar charts.This time the drop off in potency is less pronounced as the origin of the attempt moves wider,but the same effect is demonstrated.The previous graph are of interest from an attacking perspective,but one way to gauge the defensive contribution of a team or player or identify a defensive tactical approach may be to see if teams are blocking more shots than is usual.

How the Likelihood of Shots Being Blocked Decreases as Shooters are Forced Wide.

When it comes to blocking shots defensive players are much more adept at getting in the way of longer ranged efforts.At least a third of longrange attempts receive some kind of intervention before they have a chance to threaten the goal keeper and blocking becomes increasingly difficult as shooters approach and enter the box.Perhaps defenders and attackers are aware that a player intent on blocking has to commit to the action and so the contest closer to the goal has become more about defenders staying on their feet and attackers maintaining possession until a clear opportunity arises.

These tentative first steps may ultimately produce goal expectancy values for every region of the pitch and a method to evaluate both attackers and defenders.Numbers of players present around the ball,division of scoring method (head or feet) and even which side of the goal and which foot was used for the shot are obvious parameters that will change the raw prediction of success.But for the moment they can be used to give a raw flavour of how likely was an attempt to result in a goal.Two EPL Goal of the Season candidates stand out this seasons.Cisse's "wrong footed" strike against Chelsea and Crouch's acrobatic effort against the Champions.

Using the co ordinates of Cisse's shot and the regressions,an effort from that position on the pitch would result in a goal around once in every 100 attempts.Crouch's effort scores at about the same frequency.So we can't split the goals in term of how unlikely they were.The skill required to execute the attempts were also comparable,although Crouchie was much more involved in his own set up.Cisse's certainly had the wow factor and Crouch's had Stoke's trademark long ball game mixed in with skilful one touch control.Sometimes even advanced analysis can't split players,ultimately we sometimes have to fall back on aesthetics.

However,we can say with certainty that Crouch's goal is a long way from being the most unlikely scoring event seen at the Britannia Stadium.That accolade goes to Wigan's Maynor Figueroa whose quickly taken free kick from inside his own half would produce a goal once in every 3,000 or so efforts.

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