Are the All Black Selectors biased against Canterbury?

This is a guest post by Scott Brooker.

Does the Canterbury rugby team's recent run of success suggest that the All Black selectors are biased against Canterbury players? I don't know. But it's certainly an interesting question. As a result of their away victory in the final against Wellington last Saturday night, Canterbury has now won six national rugby premierships in a row. The NZRU keeps changing the competition format on a regular basis, but at a minimum there have been seven teams who are in the top division and therefore eligible to win the premiership each year. These teams are generally strong, and one weak team at least has the potential to get relegated to the lower division in any given year. To win six championships in a row is an exceptional achievement. It's easy to conclude that Canterbury is simply a very strong team, but there are at least a couple of factors that should naturally assist with spreading the talent pool more evenly and therefore contribute to making it difficult for the same team to win year after year:

• The ITM cup is run at the same time as the best players in the country should be in the All Black environment. This can take as many as 30 players out of the ITM cup at any one time. The number of players who would be considered All Blacks who are playing in the ITM cup is few and decreasing every year. Very good players should be selected for the All Blacks, which should disproportionately hurt the chances of winning of the provinces with the better talent base.

• It could be argued that the above only affects the very top players, but there is another factor that affects the rest of the player base. It is reasonably common for players to shift to different teams from season to season. A common reason is to get more opportunity. If there are too many good players that play your position in your current team, you think about moving to a team that is weaker in that position, thereby weakening your former team and strengthening your new team. This generally occurs between seasons.

Neither of these factors would perfectly balance out the competition. But they would assist with increasing the probability of a different winner each year, and making six wins in a row very unlikely.

So what are the candidate explanations for Canterbury's consistent success? I can think of a few:

1. Pure luck. Although winning the title by luck alone six years in a row must be extremely unlikely.
2. As inferred in the title, perhaps All Black selectors are for some reason more reluctant to pick Canterbury players than players of similar ability from other provinces, which would  diminish the effect of the first bullet point above. They might require a longer period of good performance to be picked, or they may be ignored entirely. I'm not really suggesting that the All Black selectors sit there with their clipboards and put big red lines through player names simply because they play for Canterbury, but it is possible that Canterbury selects players with particular attributes (eg. Goal kicking ability, percentage of missed tackles etc) that contribute to success which are undervalued by the All Black selectors.
3. Canterbury is a fantastic place to live, which encourages players to stay here regardless of their selection opportunities. The counterpoint to this one would be that Canterbury has probably never been a more difficult place to live than it is currently.
4. Canterbury is no better than other teams in terms of individual player ability, but has a team culture that leads to a more cohesive style of play and helps the team to win more often. A counterpoint to this would be that with player movements between provinces, other teams have an opportunity to learn and improve their own team cultures.
5. Canterbury is no better than other teams in terms of individual player ability OR team culture, but has uncovered a secret formula for how to play in play-off matches. If we assume that the team that finishes top in round-robin play is likely to be the best team, then Canterbury has been the best team in only three of their six successes. In the other three successes, they played the top qualifier in the final and had to play the final away from home.
6. Referee bias in favour of Canterbury (unlikely as their performance is assessed).

What other possibilities are there to explain this run of success? With a well-designed framework, some of these are probably testable using variables including player movement rates, All Black selection rates and how long a player lasts in the All Blacks after initial selection. Any other ideas on how to distinguish these hypotheses in the data are welcome.