As some of you may have read in other threads, I have a strong interest in the NCAA’s Ratings Percentage Index (RPI) as applied to Division I women’s soccer. The Division I Women’s Soccer Committee makes all decisions about at large selections for the NCAA tournament, makes all seeding decisions, and makes all game siting decisions. The RPI is the only statistical rating system and the only ranking system that the Committee uses in making those decisions. Starting about two years ago, in trying to understand some of the Women’s Soccer Committee’s at large selections and seeds, I began to wonder whether the RPI is able to accurately rank teams from different regions in relation to each other, given the limited number of inter-regional games. I felt it would be possible to develop hypothetical scenarios to answer my question. In order to run these scenarios, I used Excel to program the RPI formula, entered the individual games data for a number of hypothetical scenarios, and then determined the RPI ratings for the teams in those scenarios. In addition, I entered all the real individual games data for the 2007 season so that I could determine the RPI the Women’s Soccer Committee used in making its tournament decisions. (The NCAA published some RPI rankings during the season and also published its RPI rankings based on all games including the tournament games. The NCAA did not, however, publish either its RPI ratings as distinguished from rankings or the RPI rankings the Women’s Soccer Committee used.) Finally, I ran a further hypothetical scenario based on the 2007 real individual games data with modifications, to test whether what my other hypothetical scenarios showed would apply to a real NCAA Division I women’s soccer season. Based on this work, I prepared a paper on the RPI. I am providing, for those interested in this topic, a copy of the paper in pdf format as an attachment to this entry. I also am attaching, as a separate pdf file, the NCAA staff’s rationale for using the RPI as issued in a Frequently Asked Questions format. I am including that FAQ since I refer to it in the paper. What I am hoping to generate in this thread is serious conversation about the NCAA’s use of the RPI for Division I women’s soccer. I’m hoping people will read the paper, comment on it, critique it, suggest added or modified substance for it, and add your own substance proving the paper's conclusions right or wrong. I’m also hoping people will pass the paper on to others interested in the Women’s Soccer Committee’s decision-making process. I’m also hoping that people will not use this thread just to vent their feelings (pro or con the RPI, or the Women’s Soccer Committee’s decision-making, or the paper’s conclusions), although I realize that anyone can say whatever they want. I have provided the paper to all members of the Women’s Soccer Committee, to all members of the NCAA’s Championships/Competition Cabinet (which oversee’s the Committee), to the NCAA in house statistical staff people who are involved with the RPI as used for Division I women’s soccer, to all Division I women’s soccer coaches of West Region teams (I live in the West Region and am a Portland Pilots fan), to the publishers of the SoccerRatings and Massey ratings, and to various soccer media. My hope is to generate a serious look by the NCAA, as well as by the Division I women’s soccer “public,” at the way in which the NCAA uses the RPI for Division I women’s soccer, with a view towards the Women’s Soccer Committee improving its decision-making process. The following is a short summary of the paper. (The paper itself is 53 pages and the NCAA’s FAQ is several additional pages.) In Part 1 of the paper, I describe how the Division I Women’s Soccer Committee uses the RPI in making decisions for the Championship tournament, how the NCAA computes the RPI, and what the NCAA’s rationale is for using the RPI. In Part 2, I describe two serious problems with the RPI. In Part 3, I describe the limitations the Committee should impose on itself in using the RPI, due to the RPI’s problems. In an appendix, I describe some lesser problems with the RPI and suggest some changes the NCAA could make to address those problems. The central part of this paper is Part 2. Part 2 demonstrates that the RPI, as a rating system, has two very serious limitations: First, the RPI is not able to compare teams from one region of the country to teams from other regions if there are differences in strength among regions. And, in fact, there are differences in strength among regions and they are significant. As a result, the RPI unfairly discriminates against teams from strong regions and unfairly discriminates in favor of teams from other regions, and significantly so. In 2007, the data demonstrate that the West Region was significantly stronger, on average, than the other regions, and the RPI caused the Committee to discriminate against it significantly in its selection of at large teams to participate in the Championship tournament. This does not mean that the RPI always is biased against the West. Rather, it means that the RPI is biased against the strongest region or regions, whichever ones they might be from time to time. In 2007, the data simply demonstrated that the West was the strongest on average, to a significant degree, so that it was the region that suffered. Second, the RPI is quite inexact. For example, it cannot reliably distinguish between the team it rates as #16 and the team it rates as #40. It cannot reliably distinguish between the team it rates as #47, which is roughly the “bubble” point for selection for an at large position in the Championship tournament, and the team it rates as #90. Even expecting this level of accuracy may be overly optimistic. Since the NCAA appears committed to continued use of the RPI, I do not suggest in the paper that the Committee not use the RPI. However, given the RPI’s serious problems, I urge that the Committee understand the problems and be very disciplined in limiting how it uses the RPI. Part 1 of this paper suggests that the Committee’s present practice is to rely heavily on the RPI, much more so than is appropriate and in a way that has made its decision making process unfair. If that is true, then the Committee will need to make major changes in its decision making process if it is committed to fairness.