Tuesday, 19 March 2013

Ranking Points and Rugby World Cup Groups.

Step forward Italy. Not only did they record two victories in the recent rugby Six Nations competition, they also held England to a narrow seven point victory at Twickenham on the penultimate weekend. Thus ensuring that England's visit to Wales was not just about the hosts denying England the Grand Slam. The destination of the title was also very much up for grabs.

By holding England to their narrowest home winning margin against Italy, the Azzurri gave Wales the chance to win the title if they defeated England by at least seven points, assuming the Welsh maintained their overall try supremacy across the tournament as a whole. England went into the final game as narrow three point favourites, implying that they were almost a converted try superior to their hosts. But with the roof of the Millenium Stadium closed and passion aplenty, Wales pummeled England into first half submission, before drawing well clear in the second period with a brace of Alex Cuthbert tries.

Wales and England will of course meet again in the 2014 tournament, but an even more eagerly awaited rematch takes place in the opening group phases of the 2015 World Cup. The Rugby World Cup initially consists of four groups of five teams, so the meeting of two highly ranked teams at this early stage isn't particularly unusual. But as Simon Gleave points out here, Group A also contains Australia. So one of the currently 3rd, 4th and 5th rank sides in the world will fall before the knockout stages.

The draw for the 2015 competition took place on the 3rd of December 2012, a year after the conclusion of the 2011 tournament in which Wales finished 4th. So how did Wales return from the World Cup in New Zealand seeded inside the top eight, rise to fifth at the conclusion of the 2013 Six Nations, having retained the title they also won in 2012, but found themselves outside the seeded top eight for the all important 2015 world Cup draw which took place in December 2012.

48 hours before the draw, Wales lay in 7th place in the IRB rankings in front of Samoa and Argentina, who were 8th and 9th respectively and separated by the fourth decimal place of their rating figure. Nether of Wales' rivals were playing prior to the draw, but Wales entertained 3rd ranked Australia.

Crucially for Wales, the IRB ratings operates on a "points exchange" system, where the winning team always takes rankings points from the team they defeat, with the defeated team losing the same number of ranking points. In addition margin of defeat or victory is only accounted for where the margin is greater than 15 points. So a narrow defeat, even against one of the best teams in the world will always see the defeated team's ratings fall.

Australia defeated Wales on the game's last play, scoring 26 seconds before the clock turned red, the margin of defeat was just two points, although it may as well have been 15 and with Samoa and Argentina idle, Wales slipped to 9th in the seedings and into the 2015 Group of Death, ironically alongside Australia.

A more satisfactory approach would be to use a least squares or power rating approach, where unbalanced and incomplete schedules are accounted for and margins of victory or defeat are evaluated with full regard for the quality of the opposition faced. Teams which perform well, even in defeat against superior opponents aren't automatically penalised under this method and collateral form lines quickly appear between teams which rarely meet through results achieved against common opponents.

In short, ratings are created that best explain the observed match scorelines in previous contests and the difference in the ratings of two teams can be used to form a view of the likely margin of victory in any future matches.

From the start of 2011 to the 2015 draw in December 2012 Wales played 26 games against teams rated inside the IRB top 12 including 6 matches against Australia, each of which they lost, albeit usually by just a point or two and often in the dying seconds. Argentina played 16 such games and Samoa just 6. So not only is sample size an issue, but Wales, by testing themselves often and against the very best lay themselves open to leaking IRB ranking points by playing close, but losing encounters against the Southern Hemisphere's best. Had the final match against Australia, just two days before the draw, not taken place, Wales would have remained in the all important second pot of seeds.

In order to compare the the IRB ratings method, where margin of victory is ignored until it reaches an arbitrary 16 points, to a least squares based approach where m.o.v. is continuously applied, I calculated power ratings for international rugby sides firstly, from the start of 2011 up to the date of the draw on December 3rd 2012 and also post World Cup 2011, a period replete with narrow Welsh defeats.

Power Ratings Incorporating Strength of Schedule and Margin of Victory or Defeat.

Team. Power Rating. Post 2011 World Cup until 2015 Draw. Power Rating. Start 2011 Year until 2015 Draw.
New Zealand. 27.9 31
France. 16.2 18.6
South Africa. 15.9 18.4
England. 14.3 17.6
Australia. 9.9 16.5
Ireland. 9.2 13.3
Wales. 8.9 15.3
Samoa. 8.2 13.1
Argentina. 3.3 6.1
Scotland. 1.4 8
Tonga. 0.9 2
Italy. 0 0

Wales were seeded in a comfortable eighth position under the IRB rankings at the conclusion of the 2011 World Cup, well clear of the Samoa. By December 2012 Samoa had overtaken them and escaped from pot three under the IRB rankings. Under the official rankings, Wales had leaked ranking points through narrow defeats to good sides and Samoa had largely remained static through inactivity, coupled to two narrow wins.

It's hard not to conclude that Wales' ranking suffered because the IRB ratings over value and are too sensitive to simply winning and ignores performance, even in defeat. Both the tabulated methods above that account for strength of schedule and margin of victory, keep Wales comfortably within the top eight nations.

So for almost two years prior to the 2015 draw and even during their poor run, post WC 2011, the Welsh had proved themselves superior to both Argentina and Samoa when actual scorelines took a more prominent part in the calculation.

England discovered that margins matter when they defeated Italy by only seven points on the penultimate weekend, handing a lifeline to Wales in this year's Six Nations and margins should probably also play a more prominent role in deciding World Cup seedings.

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