One of the most basic questions people ask about players and teams is, "How good are they on the offensive and defensive ends of the floor?" There are many ways to attempt to answer such a question, but there is one metric in particular that gets right to the heart of it: Offensive Rating - ORtg and Defensive Rating - DRtg.
Offensive Rating is an estimate of the points a player produces for his team per 100 possessions, or, the points an entire team produces per 100 possessions. Defensive Rating uses precisely the same concept, but focuses on points allowed rather than points scored.
Dean Oliver was the first to introduce the Offensive/Defensive Rating metric to the world of NBA analytics. It's a fairly simple idea, but one that does a good job of summarizing a player's contribution on offense/defense. It is a handy way to summarize team effectiveness on both ends of the court as well.
How Offensive/Defensive Rating are Calculated
As we stated, the overall concept is not complicated. Offensive Rating, for instance, is found by taking points scored and dividing that number by the total number of possessions. This value is then multiplied by 100 to get a per 100 possession result.
The difficult part of this formula is deciding how we will define possessions. A separate formula has been developed that estimates the total number of possessions by a team. There is a multiplier used to account for the fact that some free throws taken are the result of a possession, while others (such as "and-one" possessions) are part of an already existing possession.
Other adjustments include eliminating offensive rebounds from consideration (statisticians consider these rebounds an extension of the previous possession, not a new possession) and adding a multiplier to adjust for team rebounds.
How to Interpret Offensive/Defensive Rating
Offensive/Defensive Rating has by far the most meaning when we have knowledge of other players' (or teams') ratings for comparison, and/or know what the league-wide averages are for individual players and teams.
On the high end of the scale, we can take a look at some of the best of these ratings in NBA history. The highest career Offensive Rating among retired players is Reggie Miller's 121.5. The best career Defensive Rating in NBA history is 95.3 (Gar Heard).
In a recent NBA season, the average Offensive/Defensive Rating was 104.6. These numbers give us a guideline on how to interpret a range of values for this metric.
Benefits of the Offensive/Defensive Rating Statistic
This metric is a substantial improvement over looking at per-game averages for players and teams. Offensive/Defensive Rating is a per-possession statistic, so no matter how many possessions per game a team has, this metric compares them on an "apples to apples" basis.
For example, a team that plays at a high pace (tends to shoot early in the shot clock, runs the fast break often) will have more possessions per game than a team that uses more of the clock and plays a lot of half-court offense. Comparing these teams' per-game numbers gives the higher-paced club an unfair advantage, while using per-possession statistics levels the playing field and paints a much more accurate picture.
Negatives of the Offensive/Defensive Rating Statistic
The primary failing of Offensive/Defensive Rating is that it is an overall view of a team's or player's performance on both ends of the basketball court. While that is useful, it doesn't tell us anything about why that team or player has attained that level of production. It's a good summary of performance, but if one wants to look any deeper than that, more research is needed.
Offensive/Defensive Rating - Behind the Number
The first thing that must be done when performing this calculation is to determine the number of team possessions. The formula for this is:
The 0.96 multiplier and 0.44 multiplier are the ones discussed earlier as adjustments for team rebounds and free throws that do not use up their own possession, respectively.
Frankly, that's the hard part. Now that we have a value for total possessions, we can plug that into the following formulas:
Offensive Rating = points scored * 100/total possessions
Defensive Rating = points allowed * 100/total possessions
Before there were advanced NBA statistics, we all used per-game averages to evaluate players and teams. The next step was to find a method to make these judgments in a much more accurate and meaningful way. Metrics like Offensive Rating and Defensive Rating have become that next level of analysis. Players who score 25 points per game or teams who score 105 points per game may appear impressive on the surface, but when we lift the hood and look underneath, things may not be as they seem.