Relative Percentage Index aka RPI is widely used to produce power ratings. Unlike most power rating systems, the RPI does not consider the margin of victory or how well a team has played. It only considers whether a team won or lost. In fact, the RPI is based entirely on the winning percentage. It essentially measures a team’s strength of schedule and how a team does against that schedule. It is based 25% on a team’s winning percentage, 50% on the winning percentage of their opponents, and 25% on the winning percentage of their opponent’s opponents.
For example if;
Atlanta is 4-1 (W/L Pct= .800)
Atlanta’s opponents averaged a 12-8 (W/L Pct.= .600) record
Atlanta’s opponents’ opponents averaged a 64-16 (W/L Pct = .800) record, then,
RPI for Atlanta is = (0.25 * .800) + (0.5 * .600) + (0.25 * .800)=.700
It is best known for its use in college sports. Rarely is it mentioned in NBA circles because all teams play each other and most of the time teams have fairly similar strength of schedules.