Worlds Top 50 G1 Races which rates international Group/Grade 1 races by calculating the average World Thoroughbred Ranking (WTR) ratings of the first four placed horses in each race over the previous three calendar years. For jurisdictions with a season that does not follow a calendar year, such as Hong Kong, the WTR ratings achieved during the corresponding seasons are used.

http://ifhaonline.com/resources/2012Rankings/Top50_G1_Races.pdf

Thanks for sharing, makes an interesting read.

really good read

Personally a bit irrelevant currently as the ratings for various races have been skewed due to the presence of frankel/black caviar so could be a different picture in a few years time

Well, who knows about the quality of horses in years to come? It's just one way of trying to rank the world G1's, and I think it holds up pretty well, even if it isn't an exact science.