Here is an explanation.
This is another stab at something I've worked on for years -- the idea is to take the performance for each team in the Four Factors, and find teams with similar profiles in the past to determine how teams of that ilk eventually do in the playoffs.
Four Factors
How do basketball teams win games? While searching for an answer to that question, Dean Oliver identified what he called the "Four Factors of Basketball Success":
1. Shooting (40%)
2. Turnovers (25%)
3. Rebounding (20%)
4. Free Throws (15%)
The number in parentheses is the approximate weight Mr. Oliver assigned each factor. Shooting is the most important factor, followed by turnovers, rebounding, and free throws. [Editor's note: I agree with the order, but disagree with the weightings.] These factors can be applied to both a team's offense and defense, which in a sense gives us eight factors. Let's take a closer look at how these factors are measured, using the 2004-05 Phoenix Suns as an example.
Shooting
The shooting factor is measured using Effective Field Goal Percentage (eFG%). The formula for both offense and defense is (FG + 0.5 * 3P) / FGA. For the Suns offense this is (3351 + 0.5 * 796) / 7018 = .534, and for the defense it is (3328 + 0.5 * 494) / 7485 = .478.
Turnovers
The turnover factor is measured using Turnover Percentage (TOV%). The formula for both offense and defense is TOV / (FGA + 0.44 * FTA + TOV). On offense, the Suns turnover percentage was 1125 / (7018 + 0.44 * 2080 + 1125) = .124, while on defense it was 1131 / (7485 + 0.44 * 1775 + 1131) = .120.
Rebounding
The rebounding factor is measured using Offensive and Defensive Rebound Percentage (ORB% and DRB%, respectively). The formula for offense is ORB / (ORB + Opp DRB), while the formula for defense is DRB / (Opp ORB + DRB). In 2004-05, the Suns offensive rebound percentage was 967 / (967 + 2550) = .275, and their defensive rebound percentage was 2652 / (1233 + 2652) = .683.
Free Throws
The free throw factor is a measure of both how often a team gets to the line and how often they make them. The formula for both offense and defense is FT / FGA. The Suns free throw rate on offense was 1556 / 7018 = .222, and on defense it was 1320 / 7485 = .176.
Here's the methodology:
1. Calculate the Four Factors for each team on offense and defense (technically, I guess that would make it Eight Factors, but whatever).
2. Calculate the Z-score for each team's factors by subtracting the league average and dividing by the league's standard deviation.
3. Compute the difference between the two teams' z-scores and square it... Do this for all 8 factors.
4. Add the squared differences together, weighted by the following: Offensive & defensive eFG% --> 0.2 each; offensive & defensive TOV% --> 0.125 each; offensive & defensive ORB% --> 0.1 each; offensive & defensive FT rate --> 0.075 each.
The result is how similar the two teams are in terms of not only how their factors match up, but also by placing more weight on the factors that correlate most to team performance. Here are the results for the 2010 teams so far:
