The defensive rating is based on what we call stop percentage, which is a statistical approximation of the percentage of defensive possessions where the player helps create a turnover, missed field goal, or missed free throw. This approximation is based on certain player stats (minutes, steals, fouls, rebounds and blocks), the player's position, certain team statistics (for and against), and a factor for league averages for that season.
We have also incorporated NBA and ABA All-Defensive team selections as a bonus for those players.