The Rating Percentage Index, commonly known as the RPI, is a quantity used to rank sports teams based upon a team's wins and losses and its strength of schedule.
In its current formulation, the index comprises a team's winning percentage (25%), its opponents' winning percentage (50%), and the winning percentage of those opponents' opponents (25%). The opponents' winning percentage and the winning percentage of those opponents' opponents both comprise the strength of schedule (SOS). Thus, the SOS accounts for 75% of the RPI calculation and is 2/3 its opponents' winning percentage and 1/3 times its opponents' opponents' winning percentage.
(From http://en.wikipedia.org/wiki/Ratings_Percentage_Index)
