Tuesday, 28 January 2014

How Rugby League Teams Win and Lose

This blog occasionally covers sports other than football, so following on from Sunday's look at the relative abilities of NFL and rugby union kickers, I've plucked some low hanging fruit from union's first cousin, rugby league.

For those more familiar with union because of the extensive media attention it receives compared to league, I'll run through some of the differences.

Union and league split from a common ancestor at the dawn of organised sport and while the present day sports are visibly related, the differences are far deeper than simply the head count. 15 players for union, two fewer for league.

The tackle area is where the deepest division has occurred. Union allows for players to compete for possession, whereas league quickly ends the play, invariably allowing the team in possession to continue their attack and instead relies on a tackle count, similar to the four downs in the NFL, as a way of turning the ball over to the defence.

Scrums are contested, sometimes interminably in union, but they merely provide a means to restart the game and free up space in league.

The method of scoring is where a shared heritage is most evident, tries and kicks are identical under both codes, although inevitably the points awarded varies.

It has been off the field where the biggest interaction and division between the two closely related sports has occurred, especially in the modern era. League embraced professionalism by paying their players, while union maintained an amateur ethos, in theory, if not entirely in practice.

Therefore, movement of talent, primarily from union to league in return for a wage and a threatened lifetime ban was a feature of the two sports in the latter part of the last century. However, once professionalism became inevitable in union as well, the talent drain was largely reversed.

Home unions paid out large sums of money to attract league talent, such as England's pursuit of Andy Farrell, although the skill premium from the deal largely skipped a generation to his son, Owen. On the pitch, at least.

As with NFL and union, kicking allows for the most natural comparison between the two codes of rugby, assuming that data is available.

Stephen Myler came from a long line of distinguished league players. But swapped Widnes Vikings and Salford City Reds not for the Saints of St Helens, but the Saints of Northampton in union's Aviva Premiership, where his kicking ability and game management with the boot is perhaps better suited.

Northampton's Stephen Myler prepares to impose league bred tackling on the Lion's captain. 
Myler the union kicker verses Myler the league player, will have to wait until I've collected enough data. In the meantime, data for rugby league is as rare as it is for union. Although, the small amount available on the net does have relatively large counts, at least for individual teams and much of it can be converted to rate statistics.

For example, the number of clean line breaks per carry or the more familiar conversion rates for kickers.

If we assume that sample size can at least partly smooth out the lack of detailed information, such as pitch position and opposition strength, we can attempt to answer three important questions relating to such data.

Firstly, is there a difference in the rate figures for on-field actions across teams. Next, how do these figures correlate with success over a season and finally, are the figures broadly repeatable for sides from season to season.

For example, over the last five completed season, Super League sides attempted a combined 378,500 carries, producing 10,553 clean breaks. Overall, a league average 2.8% of carries resulted in dangerous, clean line breaks.

Even if each side was equally adept at making clean breaks, the percentage of such breaks wouldn't necessarily be exactly 2.8% per team over a season. Random variation and different numbers of carries would combine to give a spread where transiently "lucky" sides were above the average and "unlucky" ones ended up below par.

So a simulation of the percentage line breaks obtained from typical carry numbers, but assuming each side was equally talented, will show still show a spread of results.

However, the reality from the previous 5 Super League seasons shows an even bigger spread compared to such simulations. Therefore, we can probably conclude that making a clean line break in rugby league is a talent that isn't shared equally across all 14 Super league teams.

The best exponents of this skill over the last five seasons was the 2012 Wigan Warriors with a likely true clean break rate, once random variation is stripped away, of just under 9 per 200 carries and the 2010 Castleford Tigers the poorest with just over 3 per 200 carries.

The skill or lack of it is also reasonably persistent from one season to the next. R^2 values for year n against year n+1 for the percentage of line breaks per carry is around 0.25, so 25% of the variance in clean breaks survives from one year to the next.

Lastly, the skill is also reasonably strongly correlated to success over a season. A side with a greater percentage of clean breaks can also expect to have greater numbers of wins.

In short, producing clean breaks in rugby league is a talent that is unevenly spread across Super League sides, it partly persists across seasons and correlates well with success. Overall this suggests the rather obvious conclusion that players that can produce line breaks are a valued rugby league asset.

We can use this technique to look at other on field events. Offloads, for example, where a player passes to a colleague as he is being tackled can produce big gains in territory as defenders are committed to the tackle area and may relax in anticipation of the tackle being completed.

The spread of the rates at which teams pass out of the tackle is certainly wide enough over the last five seasons to suggest that the observed rates aren't the product of natural variation around a common mean. Tactically, or through desperate necessity some teams appear to attempt this high reward, but high risk play at higher rates than others. It is also a tactic or play that sides stick with from season to season, but it is also completely uncorrelated to success or failure in terms of match results over the season.

Below, I've listed a few other match events from rugby league and how they each match up in terms of likely skill differential, repeatability and correlation to success.

Rate of On Field Actions. Size of the Rate Difference seen between sides. Reproducable Over Seasons. Correlates to Winning/Losing.
 Missed Tackles. Very Large. Reasonably Strong. Fair Correlation to Losing.
Runs From Dummy Half. Very Large. Reasonably Strong. Uncorrelated.
Offloads in Tackle. Large. Strong. Uncorrelated
Clean Breaks. Reasonably Large. Reasonably Strong. Strong Correlation to Winning.
Goal Kicking. Small. Weak Uncorrelated.

Preventing line breaks and the importance of an extremely resilient defence appears to be highlighted by the correlation of missed tackles to losing and clean breaks to winning. It is also worth noting that the most significant league to union cross over of recent years was the success enjoyed by Shaun Edwards, primarily as a defence coach, first at Wasps and later with Wales. His ideas have transformed the teams he has been involved with.

Although there will always be ebb and flow, the influence of league over the way union has developed in recent years would appear to be significant.

No comments:

Post a Comment