At the heart of the rating system used to rate Scrabble
players in Australia is a rule which models the probability that a
higher rated player will beat a lower rated player. The curve is a
function of the difference in ratings. Prior to Dec 1, 2006, Australia
used a logistic curve with a slope parameter of 172. The formula
(expressed in spreadsheet language) is
We now use a straight line rule which better accords with the prior data. Your percent chance of winning is fifty plus one twelfth of the rating difference (but capped at 95%).
The practical consequences are that the higher rated players in a section will find it fairer in maintaining their rating, or being able to progress to the next higher sectionif they can prove their worth.
Ratings will slowly change as the result of the change, and it
is possible that we may again get a mismatch between the observed win
proportions and the modelled probability. Monitoring will occur from time to time by a working party assigned to CASPA.
Quick rating check
Here’s an example of how you can calculate your rating change.
Example: You are rated 1453. You win 5 out of 7 games against opponents whose average rating is 1393.
Enquiries regarding the National Rating System may be sent to the National Ratings Officer, Martin Waterworth, at firstname.lastname@example.org.