Friday, September 26, 2014

Offensive and Defensive Rating & Win %

Looking at team data, I was interested in understanding how effective team offensive and defensive rating serve as a measure of success. Offensive rating measures a team's points scored per 100 possessions. Conversely, defensive rating measures a team's points allowed per 100 possessions. To measure success I used regular season win %. I pulled historical team data from the 1950-51 through the 2013-14 regular seasons from Because doesn't provide raw data for analysis, I used excellent code I found here*, which allowed me to loop the URLs for all teams for all seasons.

Before I simply plotted individual team offensive and defensive rating, I looked at the league average for each measure over the last 63 seasons to see if there was a trend. The chart below, which plots NBA regular season mean offensive rating on the left side and mean defensive rating on the right side illustrate two things. First, the ratings mirror each other. This makes sense because for one team to have a more offensively productive season there has to be another team(s) that gives up those points. When aggregated across the league for a given year the offensive and defensive ratings become quite close (usually the difference is tenths or hundredths of a point).

Second, and more importantly, from the 1950s until the early 1980s NBA offensive production soared from about 85 points per 100 possessions to about 108 points per 100 possessions. (The NBA shot clock was introduced in the 1954-55 season so its not like a technical rule change was responsible for this increase.) This high offensive output plateaued in the 1980s and 1990s, but dropped to about 100 points per offensive possession in the late 1990s and early 2000s. By the mid-2000s it picked up slightly but even last season is about 5 points per 100 possessions off the league mean peak established in the 1980s and 1990s. (So defenses appear to be adjusting to prolific offenses.)

The chart below matters because I cannot simply plot a team's offensive or defensive rating for a given season against win % because there were teams in the 1950s and 1960s that had a  high win % but had a much lower offensive rating than teams that played in the 1990s (that had a low win % but had a higher offensive rating) on account of the offensive zeitgeist of the 1990s. (In other words, I had to take into account the change in league-wide scoring over time.)

To account for the increase in scoring over time, I used relative offensive and defensive rating, which simply subtract a team's offensive/ defensive rating from the league mean for that specific season. As a result, it would be clear that a team in 1958 with a relative offensive rating of +10 (or about 100 points per 100 possessions) was better offensively than a team in 1988 with a relative offensive rating of -8 (which is also about 100 points per possession). The two plots below chart relative offensive rating (left side) and relative defensive rating (right side) against win % for all NBA teams that played from the 1950-51 season through the 2013-14 season. I included a blue regression line to help illustrate the relationship. Clearly, relative offensive rating is very positively correlated with win % and relative defensive rating is very negatively correlated with win %. Both make intuitive sense.

Looking at the plots, I was interested in those anomalies over the last 63 years. Below is the same plot on relative defensive rating but without the color and regression line. I identified those teams that were exceptionally deficient or successful. In terms of relative defensive rating, some of the worst teams in the history of the NBA were the 1998-99 Denver Nuggets, the 1981-82 Denver Nuggets, and the 2011-12 Charlotte Bobcats (the Bobcats set the NBA record for the worst win % this season). Some of the best ever were the 1995-96 Chicago Bulls, the 2007-08 Boston Celtics, and the mid-1960s Boston Celtics.

The best teams historically in terms of relative offensive rating were the 2003-04 Dallas Mavericks, the 2004-05 Phoenix Suns and the 1995-96 Chicago Bulls. Some of the most offensively deficient ever were the 1998-99 Chicago Bulls, the 1987-88 LA Clippers and the 2002-03 Denver Nuggets. It should come as no surprise that in the 1995-96 season when the Chicago Bulls set the NBA regular season record for wins (72) they were one of the best offensive and defensive teams relative to all other teams in the that season in the history of the NBA.

*One of the many amazing things about the internet are sites, like stackoverflow or github, that allow very smart people to share or explain complex coding just because they want to help people or to solve challenging problem (or, presumably--for some--for validation/ recognition).

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