Monday, 17 June 2013

Power and Placement.

Two exceptional goals graced the opening weekend of the Confederations Cup. First up was Barcelona bound, Neymar's stunning right foot strike from the edge of the box. The shot's upward slice taking the ball well out of the reach of the Japanese keeper, yet still retaining enough raw power to reach the target with the minimum of fuss. Pirlo, Italy's bearded midfield maestro then emulated Neymar's execution by dispatching a free kick from a similar distance into the Mexican keeper's top right hand corner of the net.

A striker friendly ball may have contributed to the keeper's discomfort, but both players also demonstrated the kind of technical excellence required to score in the top class, world arena.

Although the former goal came from open play and the latter from a set piece, both strikes had many shared characteristics. Both were shots originating from roughly the same position on the pitch, were struck with reasonable power and crossed the goal line at similar heights and positions relative the the bar and post. If the pace of either shot had been reduced or the placement was at a more manageable height for the keeper, then the chances of the keeper pulling off a save would most probably have increased.

The origin of a shot is undoubtedly a major factor in predicting the likely outcome of the opportunity, but factors such as placement and power are also going to be significant predictors of success. In this recent post I looked at the increased likelihood of scoring from identical field positions in general open play and then from a counter attack and similarly at home and on the road. Both these cases can be used as a proxy for differing levels of defensive pressure, but we can equally try to model for more obvious differences, such as the pace and placement so noteworthy in Neymar and Pirlo's most recent efforts.

Shot placement is increasingly available from various data resellers and shot strength is at the moment defined by a subjective assessment. By including power and placement in a rudimentary shooting model we my appear to be gaining more information about the shooter. However, initially such analysis may be more applicable to the keeper because a striker may appear lethally effective when shooting at the top corner, but attempts that miss high may outweight those rarer and more memorable successes.

To illustrate the possible impact of power and placement along with shooting origin on expected scoring rates
of individual shots I looked at the effect these three parameters had on actual outcomes, along with whether the attempt was by way of shot or header.

Power and Placement Effects for Attempts from Around the Penalty Spot.

Vertical Yards from Goal. Horizontal Yards from Centre of Goal. Pace of Shot/Header. Type of Attempt. Shot Placement. Goal Expectancy.
12 0 Weak. Header. High. 8%
12 0 Weak. Header. Low. 5%
12 0 Normal. Header. High. 35%
12 0 Normal. Header. Low. 27%
12 0 Weak. Shot. High. 17%
12 0 Weak. Shot. Low. 12%
12 0 Normal. Shot. High. 55%
12 0 Normal. Shot. Low. 45%

The penalty spot is a useful constant to use as a universally recognised x,y coordinate and the table above shows how the chances of an on target effort being successfully converted alters with changing shot characteristics. Each of the five parameters were statistically significant indicators in my data set.

Although the usual proviso regarding relatively small sample sizes again applies, the wide variety of overall shot quality from identical x,y coordinates is well illustrated. A weakly hit, scuffed shot is much less likely to result in a goal compared to the type of well executed strikes we witnessed at the weekend. A shot placement to the upper area of the goal appears more difficult for a keeper to save, all other things held constant, but again a striker my waste opportunities by missing more frequently if attempting to grab an extra couple of % of goal expectancy by aiming high and missing completely.

As it was struck, Pirlo's effort would likely have scored twice every 13 attempts, if he'd merely floated it over the wall and on a similar target, the keeper would have probably saved over thirty attempts in between each concession.

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