Just about every Selection Sunday when the NCAA Men's Basketball Tournament field is announced, some pundit and insider will discuss how you can't ignore one team's RPI. But what is this strange stat? In a nutshell, the Rating Percentage Index combines a team's own winning percentage with its strength of schedule. The formula is easy, but the actual calculation takes some data mining.
The Basic Formula
The RPI is calculated by using 25 percent of the team's winning percentage; 50 percent of its opponents' average winning percentage; and 25 percent of its opponents' opponents' average winning percentage. A team with a record of 8-0, opponents' winning percentage of .600 and opponents' opponents' winning percentage of .450 would have an RPI of 0.6625. That would read: (1.000 x 0.25) + (0.600 x 0.50) + (0.450 x 0.25) or 0.2500 + 0.3000 + 0.1125= 0.6625. The most difficult part is tracking down the wins and losses of opponents and opponents' opponents' records for the calculations.
The hard math comes in the adjustments. Since 2004-05, the NCAA has weighted home and road victories. Home victories and road losses are factored by 0.6, while road victories and home losses are factored by 1.4. If a team is undefeated and plays an equal number of games at home and on the road, it does not affect the RPI. Games against your team are not factored into the opponent's winning percentage. In other words, if Team B is 7-1, with the only loss coming to Team A, then their record for Team A's RPI would be considered 7-0. NCAA hockey uses a Pairwise ranking, which is a more complicated algorithm, to set its tournament field. Pairwise, however, includes the RPI as a factor, and the NCAA adjusts for any victories that would adversely lower a team's RPI.