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GUEST BLOG: Rating Rugby Union kickers by kick difficulty - Mark Taylor

Mark Taylor (@MarkTaylor0) writes the excellent 'Power of Goals' blog.

Rugby Union, much like football is a fluid, fast moving sport, which requires a large amount of context related data to adequately describe on field events.  However, rugby also has a reasonably large array of discrete events, such as kicks at goal, where the object of the play is well defined. The quality of shooting chances in football can be calculated with reasonably clarity by using such information as the x and y field coordinates of the event and the same can be done using a logistic regression approach for penalty and conversion attempts in rugby.

OptaPro have kindly supplied kicking data from a wide range of rugby competitions from 2010 through to 2012, including English domestic league matches, European and domestic cup games, Internationals and Six Nations matches as well as domestic and Tri Nations Southern Hemisphere contests. Some of the data also included age restricted matches from the under 20's Six Nations and the IRB under 20's World Championship.

A model for the expected success rate for kicks at goal can be constructed using knowledge of the position, distance and angle from where the attempt was taken, combined with the actual outcome. We can then use this scoring expectation to not only rate the records of individual kickers more accurately by accounting for the relative difficulty of the kicks they attempted, but also chart the career development of young players. We can also begin to investigate the drop off in kicking effectiveness, both generally and individually with increasing distance and this opens up the development of in game strategies which may decide whether a side decides to kick for goal or kicks for the corner.

Common sense tells us that increasing distance from goal and increasingly widening of the kicking angle will make a kick less likely to be successful. For example, a kick, twenty metres out and in front of the posts, based on the data from 2010-2012 will be successfully kicked about 95% of the time. However, move the ball to the touchline, so that the ball is still twenty metres from the tryline and the kick becomes much more difficult. A combination of the angle and the increased actual distance to the posts now makes the kick just a 60% chance to be converted by an average kicker.

I've initially included all players in the sample in setting a baseline for kicking ability, so the group includes promising youngsters, such as Owen Farrell, who played in the IRB under 20's World Championship, as well as for the full England team during the course of the data and also includes household names in the later stages of their careers, such as Jonny Wilkinson and Andy Goode.

The analysis compares the expected success rates for kicking attempts, comprising both penalties and conversions, based on the field position for each kick. I've used an arbitrary cut off point of a minimum of 75 attempts to qualify for inclusion in the final list, because small sample sizes can often produce extreme outliers which can make players appear either much better or much worse than they actually are. Most Southern Hemisphere players are omitted from this initial analysis.

Wales' Leigh Halfpenny tops this fleeting glimpse at recent kicking ability. The data omitted the Celtic league clashes, so his figures originate from International, World Cup and European games and his number of attempts just exceeded 100.  Judged on kicking field position, an average expected return was 71 successes from 107 kicks at goal and Wales' full back and now fulltime kicker made 88, almost 25% higher than an average player from the dataset would expect to achieve. 

Table 1

Halfpenny originally took just the long range kicks for club and country and his total attempts have a much higher proportion of kicks from extreme distance than most others on the list. Over 16% of his kick attempts travelled 50 metres or more in the air and if we further break down his actual success rate compared to expectation, we see that he truly excels at these longer kicks. An average player's expected success rate for Halfpenny's kicks of 40 metres or more would have been 24 successes from 49 attempts and Wales' full back actually made an astonishing 38. Overall Halfpenny is a slightly above average kicker at distances short of 40 metres, but exceptional beyond that. The quality and range of his kicks combine to make him the top kicker. 


Halfpenny going for the posts

Table 2

The inter-play between a kicker's range and his accuracy can also be demonstrated by comparing two current Irish internationals. O'Gara is nearing the end of his international career, while Sexton is just beginning his. The numbers from the table above can help to pinpoint their respective kicking strengths and weaknesses over the last couple of seasons.

Proportionally Sexton attempts more kicks beyond 40 metres than does O'Gara and in addition, the Leinster man is much more proficient at these longer attempts. In short, O'Gara, at this stage of his career is just an average long range kicker, while Sexton is very good. However, the reverse is true at distances shorter than 40 metres, although the gap in performance is narrower than at longer distances. Both are above the group average, but this time it is the Munster man who comes out on top.


Sexton lines one up

Moving down to those who underperformed in the limited dataset, we find a mixture of seasoned kickers and young players.  Priestland's presence possibly shows how Halfpenny became Wales' fulltime kicker, rather than simply their long range specialist. The Scarlets player failed to match the average group expectation at all distances, although the figures are likely to be less extreme in a larger sample.

The presence of younger players in this under achieving selection should be expected. Age appears to be a significant factor in a kicker's development. From the data presented, a typical under 20 player would be expected to hit a 54 metre conversion 27% of the time from in front of the posts, compared to 38% for a typical Aviva Premiership kicker. The size of the talent gap begins to narrow for central kicks as we move closer to the posts and from 20 metres out the gap is down 95% for seniors compared to 91% for younger players. Tentatively we may conclude that the difference is at least partly down to less leg strength, rather than lack of talent for the younger group.

Table 3

This analysis illustrates how simple numbers can be used to enhance visual and anecdotal evidence. Kicker development can be evaluated more easily, along with strengths and weaknesses which can then be used to target improved performance.   

Perceived wisdom can also be tested. It is commonly supposed that right footed kickers are better able to convert touchline kicks from the left than from the right because the natural trajectory of the strike is towards the posts in the former case.  Using this data there is a small, but statistically significant difference between conversion rates from different sides of the pitch amounting to about a 2% improvement when a kicker kicks from his more "natural side".

We can also begin to investigate whether Southern Hemisphere kickers have a better record than a similar group of northern hemisphere counterparts, either through better skills or the advantages of occasionally kicking at altitude. Super 15 kickers did record slightly better records than their Aviva counterparts, but not by a significant amount.

The ultimate aim of such studies is to provide more informed decision making during matches. Players and teams are obviously making such decisions already, such as declining to kick for points if the kicker feels the distance is beyond his boot strength, but Opta are making quantify such decisions achievable.

If a coach had accurate information on his kicker's likely success from a certain point on the field, it is a simple process to convert this to a point expectation for that kick. For example a penalty with a 50% chance of success has a point expectation of 1.5 points. If instead the decision was made to kick to touch for a lineout near to the opponent's try line, this new phase of play would have to yield a try around one time in ten attempts to make the new decision equivalent in terms of the average expected points of the kick attempt. Such decision making processes, where the risk and reward of different decisions are evaluated and balanced are fast becoming an accepted way of coaching in similar sports such as the NFL.

Most importantly, from a spectator viewpoint, these numbers can settle arguments and by more accurately rating kickers by kick difficulty we can confirm that Jonny Wilkinson remains a kicker of the highest quality.

To learn more about OptaPro's Rugby SuperScout product, please click here

Posted by Mark Taylor at 11:42


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