Ratings Percentage Index

 
RPI (ratings percentage index) is a rating that applies to a team's record and schedule such that the stronger the opponents, 
the higher the RPI rating. Conversely, the weaker the opponents, the lower the RPI rating. In order to tabulate an RPI rating, 
each opponent's strength has to be evaluated by examining its W-L record as well as the W-L record of its opponents.


RPI =  (1/4 * team's record) + (1/2 * opponents' record) + (1/4 * opponents' opponents' record)

Records are in the usual form of a win proportion: wins / (wins + losses). Ties are treated as 1/2 win +
1/2 loss. Here is an example of the computation:


A team has played 8 of its 12 games through the season. The team's record is 7 wins and 1 loss or 0.875.

The records for the opponents are as follows:

   Team A        5   3   .625
   Team B        4   3   .571
   Team C        2   5   .286
   Team D        3   4   .428
   Team E        7   1   .875
   Team F        2   5   .286
   Team G        2   5   .286
   Team H        9   0  1.000
                --  -- 
               34  26   .545

Note that the .545 is not based on 34/60 (.567) but on the average of each opponent's percentage in the
right-hand column.

The records for each opponent's opponents are


   Team A       42  18   .750  - based on average of all of a team's opponents
   Team B       22  24   .502    and not based on total wins / total games
   Team C       18  18   .483
   Team D       32  22   .600
   Team E       30  31   .530

   Team F       29  15   .624
   Team G       37  16   .645
   Team H       39  27   .615
               ---  --    
               249 171   .594


Note that the .594 opponent-opponent figure above is not based on 249/(249+171) but rather is the average
of the percentages in the last column.

The RPI for our example team is then computed as follows:

RPI = (0.25 * 0.875) + (0.50 * 0.545) + (0.25 * 0.594) = 0.600