Pioneered by revolutionary basketball statistician Dean Oliver, offensive and defensive efficiency ratings offer a tempo-free statistical method for comparing teams on an absolute per-possession basis that removes the variable of time from statistics. Under this system, teams can more easily be compared based on the efficiency of their offense and the separate efficiency rating of their defense. Ken Pomeroy, basketball statistician, author and blogger, calculates these efficiencies for college basketball teams, and he uses this data, along with many other factors, to project game outcomes.
Calculate the number of total number of possessions for your team using the formula: field goals attempted - offensive rebounds + turnovers + (0.4 x free throws attempted) = total number of possessions for the season. This works because a possession can end only in one of three ways: an attempted field goal, a turnover or a free throw, with an offensive rebound negating additional field goal attempts.
Divide the team's total points scored for the season by the possessions you calculated in Step 1. For example, 938 total points scored divided by 998 total possessions gives your team 0.94 points scored per possession. Numbers above 1.0 are generally considered good.
Convert the offensive PPP number to an efficiency rating by simply multipling by 100. So 0.94 points scored per possession becomes an offensive efficiency rating of 94.
Use the formula field goals attempted - offensive rebounds + turnovers + (0.4 x free throws attempted) = total number of possessions for the season to calculate total team possessions.
Divide the total number of points allowed by your team by the possession total you calculated in Step 1. For example, 1009 total points allowed divided by 998 total possessions gives your team 1.01 points allowed per possession. The opposite is true for defensive PPP: Above 1.0 is bad; below 1.0 is considered good.
Convert defensive points per possession to a defensive efficiency rating by multiplying by 100. So 1.01 points allowed per possession becomes a defensive efficiency rating of 101.