the problem with sports reference "advanced stats" is you must also look at how they're calculated and what goes into their score.
The core of the Defensive Rating calculation is the concept of the individual Defensive Stop. Stops take into account the instances of a player ending an opposing possession that are tracked in the boxscore (blocks, steals, and defensive rebounds), in addition to an estimate for the number of forced turnovers and forced misses by the player which aren't captured by steals and blocks.
The formula for Stops is:
where:
- Stops1 = STL + BLK * FMwt * (1 - 1.07 * DOR%) + DRB * (1 - FMwt)
- FMwt = (DFG% * (1 - DOR%)) / (DFG% * (1 - DOR%) + (1 - DFG%) * DOR%)
- DOR% = Opponent_ORB / (Opponent_ORB + Team_DRB)
- DFG% = Opponent_FGM / Opponent_FGA
- Stops2 = (((Opponent_FGA - Opponent_FGM - Team_BLK) / Team_MP) * FMwt * (1 - 1.07 * DOR%) + ((Opponent_TOV - Team_STL) / Team_MP)) * MP + (PF / Team_PF) * 0.4 * Opponent_FTA * (1 - (Opponent_FTM / Opponent_FTA))^2
Also necessary is the calculation of Stop%, which is the rate at which a player forces a defensive stop as a percentage of individual possessions faced (essentially the inverse of Floor%, but for defenders):
- Stop% = (Stops * Opponent_MP) / (Team_Possessions * MP)
With those numbers in hand, individual Defensive Rating can be computed:
- DRtg = Team_Defensive_Rating + 0.2 * (100 * D_Pts_per_ScPoss * (1 - Stop%) - Team_Defensive_Rating)
where:
- Team_Defensive_Rating = 100 * (Opponent_PTS / Team_Possessions)
- D_Pts_per_ScPoss = Opponent_PTS / (Opponent_FGM + (1 - (1 - (Opponent_FTM / Opponent_FTA))^2) * Opponent_FTA*0.4)
So the guy who's covering the other teams best player every single game and isn't going for DRB on a team that doesn't play good defense is going to have a shit rating.
Further
- Out of necessity (owing to a lack of defensive data in the basic boxscore), individual Defensive Ratings are heavily influenced by the team's defensive efficiency. They assume that all teammates are equally good (per minute) at forcing non-steal turnovers and non-block misses, as well as assuming that all teammates face the same number of total possessions per minute.
- Perhaps as a byproduct, big men tend to have the best Defensive Ratings (although Oliver notes that history's best defensive teams were generally anchored by dominant defensive big men, suggesting that those types of players are the most important to a team's defensive success). A corollary to this is that excellent perimeter defenders who don't steal the ball a lot — for instance, Joe Dumars or Doug Christie — are underrated defensively by DRtg, and are prone to look only as good as their team's overall defense performs.