Posted by Neil Paine on August 27, 2010
Here are some quick logistic regressions I ran between offensive/defensive efficiency (as measured by my 1951-2010 estimation equation) and whether or not a team won a championship...
The first regression is between regular-season offensive/defensive rating (relative to the league average) and championships won since 1951, the first year for which I can estimate possessions. The logistic equation to predict championship probability from RS efficiencies was:
p(C) ~ 1 / (1 + EXP(4.7267572 - (0.3988116 * Offense) + (0.612137 * Defense)))
From this equation, we would expect an average team during the Regular Season (0.0 on offense & defense) to have a 0.9% chance of winning an NBA title. If you increase offense to the following levels while keeping defense average, you see this pattern:
Conversely, if you make the defense better while keeping the offense average, here are the expected championship probabilities:
As you can see, this result seems to bear out the old adage that "Defense Wins Championships"; for instance, to have the exact same title odds as a team with an average offense and a defense that was 5.0 pts/100 poss better than average, an average defensive team would have to score 7.7 more pts/100 poss than average!
However, we're taking into account all of NBA history here (well, except for 1950), and in case you forgot, there was this ridiculous defensive dynasty in the late '50s & 1960s that probably skews the findings heavily toward defense... Check out the Celtics' yearly offensive/defensive efficiency splits relative to the league average during their 11 championship seasons:
(Note: Remember, positive is good for offenses, but negative is good for defenses.)
Boston's dominance during that stretch could possibly be inflating our sense of whether defensive teams are more likely to win because: A) Their defense was so historically outstanding in the Bill Russell era; and B) Their offense was so mediocre (they were below-average in 7 of the 13 years listed above!). To avoid the possibility of this skewing our sample, let's re-run the regression using only results since the 1976 ABA-NBA merger (and using the traditional possessions formula rather than the historical estimation):
p(C) ~ 1 / (1 + EXP(5.5573404 - (0.5306148 * Offense) + (0.6129486 * Defense)))
Once again, here are the tables for average defenses (left) and offenses (right):
These findings still bear out the axiom of defense winning championships, but the split between offense & defense is much smaller than it had been when we included pre-merger seasons. Going back to our earlier example, to have the same p(C) as a team with an average offense and a -5.0 defense, an average defensive team would have to score only 5.8 more pts/100 poss than average if we use this equation.
However, the continued prominence of defense even when we drop the heavily D-oriented Celtics dynasty from the sample does suggest that, all things being equal, teams should prioritize excellence at that end of the court if they want to win a championship.