The rating percentage index, or RPI, is the universal ranking system used for many NCAA sports, including basketball. The rating percentage index combines a team’s winning percentage, opponent's winning percentage, and the winning percentage of the opponent’s opponents.
From the three statistics used, a specific formula is in place so that the RPI yields a percentage number. Currently, a team’s winning percentage counts for 25 percent of the formula, the opponent’s winning percentage counts for 50 percent, and the winning percentage of the opponent’s opponents counts for the other 25 percent. Both of the statistics other than a team’s winning percentage are used to help account for a team’s strength of schedule, so every team’s strength of schedule counts for approximately 75 percent of their RPI.
To get the exact RPI, the winning percentages are calculated, then multiplied by the numbers in the formula and added together to get a total percentage. The complete formula is a team’s winning percentage multiplied by 25 percent, added to the opponents winning percentage times 50 percent, added to the opponent’s opponents winning percentage times 25 percent. Once these three numbers are added together, they equal the rating percentage index.
The RPI plays a big role in NCAA tournament selections, so many teams purposely try to schedule tougher games, as they know it helps add to their strength of schedule, and in turn, help their RPI.