Dean Oliver’s Four Factors
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How do teams win basketball games? At the most basic level, by scoring more points than their opponents. But how do they go about doing that with the many skills needed for basketball? Dean Oliver has distilled this process down to four key factors, his Four Factors of Basketball Success:
Shooting is measured by effective field goal percentage, which is points from two- or three-point field goals divided by field goals attempted. The easiest way to calculate this is: (FGM + .5*3PM)/FGA.
Rebounding is measured by a team's offensive and defensive rebounding percentages. These are measured by: OReb% = TmOReb / (TmOReb + OppDReb)
DReb% = TmDReb/ (TmDReb + OppOReb)
This method is better than just looking at rebounds per game because teams have differing opportunities to grab rebounds based on how fast a pace they play at and how good they are at shooting and preventing other teams from shooting.
For example, the Phoenix Suns, who play at the league's second-fastest pace and are good at forcing misses, average 89.2 rebounds per game between them and their opponents, the most in the league. The Sonics, who play at a slow pace and are very good at shooting, have just 77.6 rebounds on average in their games, the lowest in the league. Thus, while Phoenix grabs 3.0 more rebounds than the Sonics per game, they give up an incredible 8.6 more to opponents and are a worse rebounding team, as their rebound percentage reflects.
Free Throws are really two skills - getting to the free-throw line and hitting them. (Danny Fortson has mastered both.) Oliver measures this by taking a team's free throws made divided by their field goals attempted, which eliminates the influence of pace.
Turnovers are measured on a per-possession basis. A possesion in basketball ends when the ball changes hand, either on a made shot, a defensive rebound or a turnover. That means the formula is TO/(FGA + (.44*FTA) - OR + TO).
While Oliver calls his system the Four Factors, it could just as easily be named Eight Factors, as each category is applicable on both offense and defense. Research by Ed Kupfer has shed light on the relative value of each factor (Kupfer did do a slightly different study, using free throws attempted instead of free throws made):
To shed more light on the Four Factors method, let's take a look at how the Sonics rate this season, their rank in the NBA, and league average for each factor (all numbers courtesy Knickerblogger.net's stats page):