Defending the RPI, Part 1 of 3

Most discussion of the RPI involves the weaknesses of the system. I haven’t seen anyone come to its defense, but the RPI is not that bad, really. Let me clarify – in the middle of December it’s bad. But the RPI is a tool in the tournament selction process, so it’s not meant to be used until March.

First, I think we can agree that the best thing the RPI has going for it is its simple formula. For those who don’t know, it’s

25% x your winning percentage (WP) + 50% x your opponents’ WP + 25% x your opponents’ opponents’ WP

More simply its 25% x WP + 75% x strength of schedule (SOS)

It’s not something one can compute in their head. But any dork with a computer can calculate the RPI. While many fans know the formula, nobody really knows how it works. Which leads me to the first of the three main complaints about the RPI:

Complaint #1: ¾ of the RPI is out of a team’s control.

While it seems like an obvious truth, that’s not how it works at all. To illustrate this let’s look at the range of values for both winning percentage and strength of schedule among all teams in recent end-of-season RPI’s.

```Year   Max WP   Min WP    Diff     Max SOS   Min SOS     Diff
2001   .9286    .0385    .8901      .6127     .4080     .2047
2002   .9286    .0385    .8901      .6099     .4064     .2035
2003   .9063    .0385    .8678      .6123     .3796     .2337
Avg.                     .8827                          .2140

```

So the portion in the team’s control has a range of values of roughly .8827, while the portion out of a teams control only has a range of .2140. Even when one accounts for the fact the winning percentage is just 25% of the formula, it still turns out to have a bigger impact than SOS:

```WP:   .25 x .8827 = .2207
SOS:  .75 x .2140 = .1605
```

SOS plays an important role, but unless your SOS is in the bottom third of college basketball, a poor schedule can be overcome with a great record. For instance last year Weber St. had a schedule ranked 178 out of 327, but with a 25-3 record they were able to have an RPI rank of 41. This is the beauty of the RPI, you can’t really schedule your way into a good rating as most people think. The more difficult your schedule, the harder it is to maintain a good record and therefore a good RPI. The range of values in the SOS is also part of the reason the RPI is useless early in the year. SOS has a much greater range this time of year, so it does control the ratings. But as the year progresses, everybody’s SOS gravitates towards .500 and winning percentage becomes more important.

Next week: Complaint #2 – When winning still hurts.