Oregon's High School Sports Source
- Written by Dan Smith
- Published on July 21, 2006
- Hits: 51549
The RPI rating is one of those computer ratings that gets a lot of press leading up to "March Madness". Here's how you decipher what's really going on.
RPI is a method of ranking teams based on the opponents that a team plays. It is one of many types of computer ranking systems that are used in the sports world. Unlike most computer ranking systems, however, RPI has a standard definition. This definition is the one that is used by the NCAA basketball selection committee as an aid to determine who will qualify for the tournament and what their seed should be. I will note that the NCAA selection committee "adjusts" this ranking, however, the exact formula is not known.
One basic assumption that must be made, is that this can only be applied to games against opponents within their own class. This is how the NCAA does it (only games against Division I opponents count). I do not want to try and make adjustments to a standard formula to account for the rating distortion that would occur from a 3A school that plays a 4A school and loses. A weaker 4A school may be at the same level as a stronger 3A school, yet the 3A school would be punished more by losing to the 4A school since a weaker 4A school will have a poorer overall record than a stronger 3A school. Try the Power Index for a ranking that tries to account for these games.
There was a lot of confusion in 2005 with regard to the NCAA's definition of RPI. Apparently, the NCAA decided to add a weighting for home versus away games. A lot of the college basketball RPIs varied for the same team, based on whether the source was using the old or new definition. I have updated to the new formula. Background information on this subject can be found here.
The RPI calculates 3 percentages and assigns each of them a weight, which is published by the NCAA. Let's say we're calculating the RPI for Portland High School (I know it doesn't exist know--I don't want to sound prejudiced towards any school here).
- Team's winning percentage. It used to be pretty simple to calculate. The new formula is a little harder. For wins, you take home wins * 0.6 and add road wins * 1.4 and add neutral wins (no adjustment). For losses, you take road losses * 0.6 and add home losses * 1.4 and add neutral losses (no adjustment). Then calculate your winning percentage using wins / (wins + losses). This % is weighted 25%.
If Portland is 2-2 with a 2-1 home record and a 0-1 road record, their adjusted wins = 2(0.6) + 0(1.4) + 0 = 1.2 and their adjusted losses = 1(0.6) + 1(1.4) + 0 = 2. Their adjusted winning percentage is 1.2/(1.2+2) = 1.2/3.2 = 0.375. They get .375 * 25% = .09375 RPI points.
- Opponents' winning percentage. This looks at the the teams Portland has played and calculates their adjusted winning percentage using the method above, excluding the games played against Portland. Let's say Portland beat Seattle and Boise. Seattle is 1-5 right now (1-4, or 0.200 in games excluding Portland). Boise is 3-2 right now (3-1, or .750 in games exlcuding Portland). When you average Seattle and Boise's winning percentages, you get (0.200 + 0.750) / 2 = 0.475. This number is then weighted 50%, giving Portland 0.2375 RPI points
- Opponents' opponents' winning percentage. This is a little trickier. Here you look at all of the teams Portland has played (Seattle, Boise, Sacramento, and Las Vegas) and calculate a opponents' winning percentage for each team. For example, Seattle is 1-5, having played 6 different teams. A winning percentage for each of those 6 teams is calculated, excluding games against Seattle (remember, this calculation is from Seattle's persective - you include all of Seattle's opponents in this, including Portland). Once you have Seattle's opponents' winning percentages, you average them all together to get a number for Seattle. You then follow the same process for each of Portland's other opponents (Boise, Sacramento, and Las Vegas). Let's say Seattle's opponents average .333, Boise's .500, Sacramento's .500 and Las Vegas' .667. Portland would then average all of these "averages" together and get .500. Once you multiply this by the 25% weighting factor, you get .125 RPI points.
- By adding up the RPI points, you get the total RPI ranking. In this case, Portland's RPI is .09375 + .2375 + .125 = .45625.
The RPI will change after each game played--it does not look at the records when the games were played, but rather what the current records are of the teams they played.