Understanding the Rating Percentage Index (RPI) in College Football and Other Sports

The Rating Percentage Index (RPI) is a metric used to rank sports teams based on their wins, losses, and strength of schedule. It has been used in NCAA basketball, baseball, softball, hockey, soccer, lacrosse, and volleyball. While RPI is not officially used in football, elements like "loss", "schedule strength", and "quality win" in the Bowl Championship Series (BCS) formula align well with the RPI formalism.

What is RPI?

The Rating Percentage Index (RPI) was adopted by the NCAA to rank teams based on their strength of schedule. The RPI calculation is critically important in collegiate athletics, when the No. 1 and No. 2 teams in the nation have few or no shared opponents. It is an attempt to adjust for quality of opponent.

RPI Components

In its current formulation, the index comprises:

Winning Percentage (WP): 25% of the RPI
Opponents' Winning Percentage (OWP): 50% of the RPI, which is the average of the WP's for each of the team's opponents with the requirement that all games against the team in question are removed from the equation.
Opponents' Opponents' Winning Percentage (OOWP): 25% of the RPI, which is the average of each Opponent's OWP. Note that the team in question is part of the team's OOWP.

The opponents' winning percentage and the winning percentage of those opponents' opponents both comprise the strength of schedule (SOS).

RPI Calculation

The WP is computed by taking the number of wins and dividing by the number of games played. As an example, consider an opponent that has a 9-1 record and that the only loss is against the team under consideration. A win over Ashland Blazer does not help the WP value used in the calculation for Ashland Blazer’s OWP, nor does a loss hurt that WP.

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OOWP is calculated by looking at each opponent and calculating their OWP, similarly excluding the games between the opponent and their opponents. If the game is a win for the team, the Winning Percentage of the game is the class weight for the opponent divided by the class weight of the team.

Changes to RPI Formula

Baseball

Recommended changes to the RPI formula have been approved by the Division I Baseball Committee and took effect at the start of the 2013 season. The new formula values each home win as 0.7 and each road victory as 1.3, a change from the customary 1.0 of the past. The new method is not quite as extreme as basketball's (1.4 for home losses and road wins, 0.6 for home wins and road losses), but the overriding factor with baseball is that the vast majority of games are played outside. The adjusted winning percentage was exemplified in a breakdown of 2011 RPI's in college baseball.

Basketball

For Division 1 NCAA Men's basketball, the WP factor of the RPI was updated in 2004 to account for differences in home, away, and neutral games. A home win now counts as 0.6 win, while a road win counts as 1.4 wins. Inversely, a home loss equals 1.4 losses, while a road loss counts as 0.6 loss. A neutral game counts as 1 win or 1 loss. This change was based on statistical data that consistently showed home teams in Division I basketball winning about two-thirds of the time. Note that this location adjustment applies only to the WP factor and not the OWP and OOWP factors. Only games against Division 1 teams are included for all RPI factors.

Adjustments

The final element of the formula is adjustments that are added, which vary from sport to sport. Overall, the adjustment made to a baseball team's RPI is about twice that made to a basketball team's RPI.

Shortcomings of the RPI

There are a few obvious shortcomings to the RPI. One is that it doesn't consider opponents' opponents' opponents or beyond. On the surface, this is a huge shortcoming since it appears to make the assumption that every team's opponents' opponents' opponents are of equal ability.

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Home and Road Games

Aside from the bonuses, the RPI makes no distinction between home and road games. Since many prominent college teams are known for playing most of their non-conference games at home, this unfairly boosts their RPI.

Head-to-Head Games

In the computation of schedule strength, a team will be counted as a tougher opponent to somebody that beat it than it will to somebody that it beat because the games between the teams are subtracted. It does become problematic when the number of games played against each opponent is not constant, however. A football team that plays one opponent twice accounts for 2/12 of that opponents' record, which is of course added twice into the OWP component for an increase of a factor of four. In other words, a game played against a team that you play twice is more important than a game played against a team you play only once.

Other Flaws

The RPI formula also has many flaws. Due to the heavy weighting of opponents winning percentage, beating a team with a bad RPI may actually hurt your RPI. Losing to a good RPI team can help your RPI.

RPI Quadrants

Since 2018, one criterion for determining selection to the NCAA Tournament has been performance against certain RPI quadrants. Typically, a quadrant 1 win is considered a "good win", while a quadrant 4 loss is considered a "bad loss".

Quadrant 1: Home games vs. RPI teams ranked in the top 30; neutral games vs. 1-50; away games vs.
Quadrant 2: Home vs. 31-75 teams; neutral vs. 51-100; away vs.
Quadrant 3: Home vs. 76-160 teams; neutral vs. 101-200; away vs.
Quadrant 4: Home vs. 161-plus teams; neutral vs. 201-plus; away vs.

RPI Replacement

The NCAA announced that the RPI would no longer be used in the Division I men's basketball selection process and would be replaced by the NCAA Evaluation Tool (NET).

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Improved RPI

The improved RPI is a self-consistent RPI-like rating scheme. There are two basic principles involved: (1) all games count equally [RPI-like] and (2) the rating of a team that goes 0.500 should equal the average of its opponents ratings [self-consistent]. Since the ratings are both input and output data in this system, the solution must run iteratively until convergence is reached.

Fixing Problems

Fixing the head-to-head and road/home problems are trivial and can be done as noted above. Since teams play most of their games against conference opponents, there is a strong correlation between a team's opponents' record and its opponents' opponents' records.

Alternatives to RPI

NET (NCAA Evaluation Tool)

When it comes time to put together the NCAA tournament brackets, college basketball’s powers-that-be love a good acronym. First there was RPI (Ratings Percentage Index), a metric that dates to the earliest years of seeded brackets. RPI eventually gave way to NET (NCAA Evaluation Tool), the sport’s signature catch-all measurement for the last decade.

WAB (Wins Above Bubble)

KPI, too, has been a frequent flyer. Now, a new metric appears poised to break contain. That’s WAB-Wins Above Bubble. WAB stands for Wins Above Bubble, and it measures-broadly speaking-the number of wins a team has against its schedule relative to how an average bubble team would fare against that same schedule.

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