Martin RPI is a rankings system that ranks all football and boys basketball teams in the state of Ohio based on their record and strength of schedule. Below is an explanation of how the formula works.

RPI: Ratings Percentage Index (WPx35%) + (OWPx35%) + (OOWPx25%) + (SSFx5%)

WP: Winning percentage. Just your good ol' wins and losses.

OWP: Opponents' winning percentage. This is found by taking the average of each winning percentage of every single team on your schedule.

OOWP: Opponents' opponents' winning percentage.  This is found by taking the average of each OWP of every single team on your schedule. This is a 2nd tier of the strength of schedule component. It's basically like saying Ohio State's 15-1 record in the Big Ten is better than Dayton's 15-1 in the A-10 because the Big Ten is a much tougher conference than the A-10.

SSF: School Size Factor. Makes it more beneficial for scheduling teams larger than you, and the opposite effect if a team is smaller. Only 5%, so doesn't make a huge difference, but it's significant nonetheless.

SoS: Strength of Schedule. This is essentially a team's RPI with their win percentage taken out. 

PPG: How many points per game a team scores.

PPGA: How many points per game a team allows.

PPG Dif.: How many points per game a team scores minus how many points per game that team allows.