This year, the Men’s Roller Derby World Cup is trying something new with its tournament format. Nothing so radical as the Roller Derby World Cup in Manchester – they’re still using Groups and Elimination to sort their teams – but they’re playing a bit with that transition from Group to Elimination Tournament in an interesting way.
Traditionally, we want to arrange the number of Groups to be a power of 2 – 2,4,8 – because this means that if we allow the top 1 or 2 to graduate in each Group, we also have a power of 2 number of teams playing in the Elimination phase. Elimination phases work most straightforwardly when there are a power of 2 teams playing, so this is a good thing.
MRDWC also wants a power of 2 number of teams in their Elimination – they want 8 of them – but there are 24 teams playing. (Originally, there were 25, which is even worse, and that situation made the following approach look like the best one.) Whilst 24 is divisible by 8, the result is 3, and Groups of 3 are rather hard to balance well; similarly, Groups of 6 (with 4 such) are quite large, and need a lot of games per Group (15, for a total of 60 games in the Group phase).
Instead, MRDWC have decided to have 6 Groups of 4…
…but this leaves them with the problem of how do you pick 8 teams to go forward, when there’s only 6 Groups?
The solution MRDWC have adopted is to allow all 6 “winners” of their Group to go through, and then supplement them with the “best” 2 of the 2nd-place teams in the Groups. (That is: of the 6 teams who placed 2nd in a Group, we try to pick the best 2 of them.) This is obviously tricky to do fairly, and the solution which MRDWC are using is to assume that the “total points difference, not counting the worst game” is a fair enough yardstick.
All of the teams have played 2/3 of their games now, at the end of Day 1, so we should have a pretty good idea of how they’re doing – and we can have a stab at using inference to predict their final games, too, to make a prediction of who will get those two extra Elimination Slots.
Where we have used actual scores, these are taken from the official MRDWC score page, linked at the top of this article.