Efficiency: NBA Offensive/Defensive Ratings for Game Handicapping and Fantasy Analysis (9/7/09)
In the last article we took a look at the most basic handicapping stat which was the ‘Possession’. The Possession stat tells you how fast or slow a pace a team plays. Higher pace usually equals higher score, lower pace usually equals lower score. The problem with the raw possession statistic is that it doesn’t actually measure quality, it only measures quantity. Any team can have a high possession statistic if they run the court and shoot in 7 seconds or less, but how good is this offense?
The way we go about determining quality from the raw possession stat is to filter it through a statistic called Offensive Rating and Defensive Rating. The statistic is very easy to compute. Simply take the number of points scored and divide it by the number of possessions used, then multiply by 100.
For example, say the final was 105-96 in the Lakers vs. Magic game. L.A. had 90 possessions and scored 105 points. What is their Offensive Rating? Simply divide 105/90=1.167. Now multiply by 1.167 x 100= 116.7. The Lakers had an offensive efficiency rating of 116.7. The league average for the 08-09 season was 106.6, so we can see that a rating of 116.7 is way above average.
In the above example you might have expected the Lakers score to be low if you simply looked at the raw possession statistic. In this game it was 90, which is well below the league average of 93.1. However, their offensive quality is high, therefore it doesn’t take L.A. as many possessions to score their points.
Defensive Rating works the same way. A teams defensive rating is simply their opponents offensive rating. To calculate L.A.’s defensive rating in this game we take the opponents number of points (96) and divide it by the number of opponent’s possessions (90) and multiply that number by 100. 100(96/90)=106.6. As you can see, L.A.’s defensive rating was 106.6, which was exactly the league average for 08-09. Orlando didn’t neccessarily play bad offense in this game, the L.A. offense was just way better.
As with Possessions, Off/Def Ratings can help you with your daily fantasy roster decisions. For example, last season the L.A. Lakers (94.3) and Sacramento Kings (94.4) played at almost the same pace. If you were trying to choose between fantasy player A playing against L.A. and fantasy player B playing against Sacramento based on raw possessions, it would likely just be a coin flip. However, if you dig a little deeper and move to defensive ratings we find that L.A.’s defensive rating last season was 104.7 while Sacramento’s was 114.7 (remember lower defensive ratings are better, while higher offensive ratings are better). What seemed like a hard decision based strictly on opportunity (possessions) has now become a very easy decision based on quality (defensive rating). While both fantasy players will likely get the same number of opportunities, the fantasy player facing the Kings will likely produce much better results based on those opportunities.
Basically you want to first calculate the number of possessions for a game, then determine the offensive and defensive ratings using the points scored and possessions stat. Once you determine the ratings, you can usually tell how the game was won, either by great defense or great offense. You can also tell which fantasy players may have better nights.
Now that we have defined and discussed some ‘macro’ type stats to begin our analysis of the boxscores, my next posts will describe some of the more ‘micro’ statistics beginning with the statistics known as the ‘Four Factors’.