In the quest to develop a statistical MMA ranking system we were drawn to RPI. The only statistical data readily available in MMA is wins and losses and an RPI measures strength based on the records of a fighter/team and the records of their opponents.

What is RPI?

RPI (ratings percentage index) is calculated as 25% of a fighter's winning percentage + 50% of a fighter's opponents' winning percentage + 25% of a fighter's opponents' opponents' winning percentage. We tweak this calculation slightly to give more emphasis on the fighter's winning percentage.

More tweaks

1. More recent fights carry more weight when determining current performance, so fights going back in time carry less and less weight in calculating RPI.

2. Inactivity, whether due to an injury or some other reason, can be a detriment to how well a fighter will perform and how relevant a fighter is to the current landscape of the sport. Many voted on rankings require that a fighter compete in the past year to be eligible for the rankings. To address this, a fighter's RPI will decay as their inactivity increases.

3. Fights in big organizations generally hold more weight than smaller shows. A fighter can go undefeated in Podunk Fighting Championship, but one win in the UFC means much more. So fights are weighed depending on the strength of the card they are fought on.

4. The final tweak is a very minor one. There is a slight tweak for split and majority decisions. It's only slight and not 1/3 (or 1/6) as it was on the judges scorecards.