Wednesday 30 January 2013

Do Rugby Kickers Have A Best Side ?

Everyone agrees on the greatest rugby union try of all time. It was scored in a losing cause, by a scratch side, contained at least one forward pass and began with " Phil Bennett covering..." and ended with "This is Gareth Edwards. A dramatic start. What a score!"

It really should have been worth more than four points.

There's less of a market for the best drop goal of all time, but with all due respect to Joel Stransky in 1995, few would look beyond Jonny Wilkinson's 2003 World Cup winning punt. As half of the Australian side loitered just on the wrong side of legality and the two hemispheres held their collective breath, Matt Dawson decided against another sniping run into the heart of the opposition defence and with just 20 seconds of extra time remaining, slung a pass back to the waiting Wilko. Positioned mid way between the centre spot and the left touchline, Wilkinson calmly curled the ball between the uprights....with his right foot.

Sports players invariably have a preferred foot when kicking a ball. From personal experience, everyone will recognise the awkward mechanics when kicking a ball, especially a dead ball, with their less favoured foot. In his final season at Arsenal, Fabregas attempted just eight shots at goal with his left foot compared to 60 with his right, a clear preference and in the same season, Van Persie used his head for scoring attempts more often than he took aim with his right boot.

So you would expect that Wilkinson would have trusted the most important kick of his England career to his natural, strongest foot. Except he didn't. While he has no doubt practiced extensively to become a two footed player (in the best sense of the phrase), he is a natural left footed striker of the ball and it is very difficult to find an image on Google where he isn't kicking with his left foot.

The reason he chose to kick the ball right footed may lie in the field position. He was probably about 20 metres in from the left hand touchline, therefore if he had used his more natural left foot, the angle of the attempt, combined with the natural expected hook on the ball would have probably seen the ball tend towards the right hand upright and potential disappointment. By using his right boot, providing the kick started inside the right hand upright, the natural trajectory of the flight of the ball would take the attempt towards the posts rather than away.

James Hook warms up with a right footed kick from the left touchline.
It was even rumoured that Sir Clive Woodward, during his brief stint at Southampton football club had encouraged penalty takers to aim for the post and allow curl of the shot to place the ball just inside the upright. Maybe a similar process was at work in the England 2003 rugby World Cup squad ?

The wider implications for Wilkinson's choice of foot is that kicking against the grain of the ball's natural in flight tendency should make right footed conversions progressively more difficult as the kick gets closer to the right hand touchline compared to the same kick attempted by a left footed player. Using conversion and penalty kicking data courtesy of Opta, I've re run the regression in this post that looked at the expected success rates of kicker by field position of the attempt. I've included another term to differentiate between attempts when the natural flight of the ball is towards the posts (right footed kickers from the left touchline and left footed kickers from the right touchline) and when it is across the face of the target (right footed from the right and left footed from the left).

Distance to Posts and from Touchline.(metres) 46 and 1. 46 and 5. 46 and 10.
Success Rate from Faourable Side. 44.4% 51.5% 60.0%
Success Rate from Unfavourable Side. 39.4% 46.4% 55.0%

The idea that there is a favourable side to kick from appears to be supported by the results of the regression. In the table above, I've used a distance of 46 metres to the posts as a yardstick and the likely success rates have been calculated from over 2000 kicks taken from 10 metres or closer to the touchline. Kickers foot preference was taken from Youtube or still images from Google.

Kicking from the preferred side from virtually the touchline increased the success rate by nearly 13% compared to potentially kicking across the face of the posts. A typical kicker would land around 39 kicks per hundred attempts from his unfavoured side, but that figure would increase to 44 if he swapped touchlines. Similar splits were recorded as the ball was moved further infield, although the improvement declined as the kick moved closer to the centre of the goals. At 10 metres from the touchline the difference between conversion rates had fallen to 9%. The chances that the difference between sides was merely down to random chance was around 1 in 50, so it is likely that we are seeing a real effect and the effect persists for all distances of kicks from near to the sideline.

As we move nearer to the centre of the pitch, statistical significance for the favoured kicking term is lost. Kicks a metre either side of a perpendicular line through the halfway line, in other words nearly dead centre to the posts, show a much reduced advantage for kicks attempted from the favoured side. However, there is a nearly 70% chance that this difference has arisen just through chance.

As far as I can tell, Wilkinson despite his right footed drop goal, always converts kicks with his favoured left. Again possibly a throwback to a static ball being harder to tame than a moving one, but it is possible that he was also playing the angled percentages when he connected with that ball over a decade ago. In short, the numbers appear to support the theory of a favoured kicking side. However, all of the theory goes out of the window, if you are as good as Dan Carter and you can do this!

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