Intro to RPI
By Brian Semonian
RauRaur's RPI Guy
12/29/2007
Come March, everyone is always talking about RPI. Last year, despite an awful Pac Ten Tournament, no one in Tucson worried about Arizona not making the NCAA Tourney because of this complex RPI thing. And although it’s early, our ‘Cats currently find themselves at the top of the RPI rankings once again. But if one asks the average college basketball fan to explain RPI, they are likely to receive a vague description at best. What is the RPI, really? Why do we take comfort every year that our RPI rating is high enough to get us into the tourney? Over the next few months, I will attempt to explain RPI as painlessly as possible for all my Wildcat brothers and sisters. Do your best to approach the subject with an attitude that would make your high school Algebra instructor proud.
The letters R.P.I. stand for Ratings Percentage Index. It consists of three parts that are all bound by a common thread, winning percentage. Understanding this specific calculation for winning percentage is the key to understanding RPI.
Normally, to figure out any team's winning percentage you take the team's number of wins and divide them by their total number of games played. For example, the ‘Cats are off to a great start this year with 8 wins and 2 losses. Their winning percentage, as usually calculated, is .800 (or 80%). Easy, right?
But this is not the winning percentage used for the RPI statisticians. For the RPI winning percentage to be calculated home wins only count as 60% of a win, road wins equal 140% of a win, "neutral" site victories are the equivalent a full win (100%) and, not shockingly, losses are worth 0% of a win. (Note the parenthesis around "neutral.” It’s important to understand that, as far as the formula is concerned, Arizona playing in Chicago's pro-Illini United Center does not count for any more the one win.)
So let’s recalculate Arizona's winning percentage using the RPI-approved metrics. To date, out of 10 games, Arizona has 7 home wins, 1 road win, 1 neutral win, and 2 losses. In other words:
So the Cats' winning percentage currently sits at .660 (not quite as impressive as once thought but nothing to complain about). Next, to determine actual RPI, winning percentage is calculated for three separate entities as shown in the formula below:
After seeing something like “Opponents' Opponents' winning percentage”, there’s reason to be thankful for websites like kenpom.com who do all data collection and arithmetic for us. Still, I think we can agree, it’s not all as complex as one might think.
As shown above, the figure that carries the most weight is “opponents' winning percentage” (worth the equivalent of “team’s percentage” and “opponents’ opponents’ percentage” combined). Because of the extra weight it carries, a team's Strength Of Schedule (or SOS) has a very direct and significant relationship with their RPI. Arizona's SOS, for example, has been the reason for their high RPI in recent years.
RPI is a great tool for fans and the NCAA Tourney Selection Committee. It allows us to look beyond a team's raw wins and losses and see the quality of their season in relative terms. Over the next few months, I will elaborate on RPI and explain Strength of Schedule in more detail. I will also break down various Pomeroy Ratings from kenpom.com. And of course, I’ll monitor how all these statistics affect the Arizona Wildcats.
Keep an eye out for the next installment of RPI Guy and enjoy tonight’s game between the teams with the two highest RPIs in the nation, Memphis and Arizona.
