Wednesday, March 27, 2013

NBA Stats: Opponent FG%

The NBA's newish resource for statistical analysis, NBA Stats, illustrates why despite the attention that Moneyball,Sabermetrics et al garner baseball as sports' source of quantitative analysis, progress in the NBA's data collection techniques(and presentation) signal the league's appreciation for analysis. The facts that there are fewer players in basketball and that more of the game is quantifiable (especially now that spatial factors can be captured) will likely arouse greater interest in NBA analytics.

The main problem with NBA Stats is that they don't share the data so you're stuck with either using their standard reports or their limited filters on larger data sets. Of course, it makes sense that the NBA doesn't share the data because they are expensive to collect (and, thus, like most other organizations that collect vast amounts of data demand compensation for sharing) and because individuals could take the data and run their own analyses and draw attention away from NBA Stats, which the NBA presumably created simply to draw more traffic to (as well as to build their brand with more quantitatively inclined consumers and fans).

A great set of individual defensive metrics that the NBA provides is on opponent's field goals (so how well all opponents shoot when being guarded by an individual player). It represents an intuitive measure of an individual's defensive success because it isolates individual defensive performance in a way that plus-minus or turnovers, which both largely reflect team defense, fail too. The metric is limited by the fact that a defender may still benefit from exceptional help defense, especially in the paint, and by the fact that some defenders' presence actually dissuades field goals from ever being taken in the first place. The data is organized in 5 foot increments so you can determine which player is a better defender closer to the basket (i.e., less than 5 feet) or on the perimeter (20 to 24 feet). 

I looked only at the 2012-13 season and focused only on defense close to the basket (less than 5 feet) and defense close to the three point line (20 to 24 feet). It's necessary to qualify based on some minimum number of FGA otherwise bench players who happened to be in when their man missed three of four close attempts appear to have the lowest opponent FG% (the lower the % suggests better defense). In fact, Terrell Harris, a 6'4" guard, has the lowest percentage of any player with opponents who have taken more than 10 FGAs (37 total for Terrell), which may reflect his superior interior defense but more likely suggests that Anthony Davis and co. help protect the net. Interestingly, of the top five defenders with opponent's that have taken at least 100 FGAs, four are on the Pacers--Roy Hibbert (48.8%), Orlando Johnson (49.4%), Gerald Green (49.6%), and Jeff Pendegraph (50.0%). (In addition, Pacers' C Miles Plumlee allows only 44.4% but he's only faced 36 FGAs.) Ronny Turiaf is the only non-Pacer with over 100 opponent FGAs within 5 feet, not on the Pacers, and allowing a FG% below fifty (49.6%). 

The Pacers' impressive interior defense likely reflects one of two scenarios. They're either the best coached and do an excellent job of executing sound interior defense, or one individual defender that gets a lot of playing time (i.e., Roy Hibbert) is such an imposing interior defender that his presence and help defense decrease not only his opponent's FG% but also his teammates' opponents' FG%. Here lies an issue with the NBA not sharing the data. To measure these two scenarios you would want to compare the other Pacers' opponents' FG% with Hibbert in the game and without him in the game. If the percentages hold in both scenarios then it suggests exceptional coaching and play. If his teammates' opponents' FG% increase without Hibbert in the game then Hibbert's value as an interior defensive player is immense.

For perimeter defense, we expect opponent FG% to be much lower as the shots 20 to 24 feet from the basket have a lower likelihood of going in than ones within 5 feet. In fact, 110 NBA players thus far in the 2012-13 season have held their opponents to under 35% FG on shots from 20 to 24 feet from the hoop. Reviewing the list of players that accomplish this low opponent FG% shows that many of them are power forwards and centers. Ideally (and NBA Stats does not let you filter by position in this table), we would want to look only at PG, SG and SF because presumably opposing PFs and Cs aren't accurate to begin with from 20 to 24 feet so low FG% may not reflect exceptional perimeter D, but rather that those PFs and Cs benefit from being able to guard poor perimeter shooters.

Three of the top five guards and SFs (not Ellington and Prince) with close to 100 opponent FGAs or more from 20 to 24 feet are surprising: Jordan Crawford (28.4%), Eric Maynor (28.8%), Wayne Ellington and Marquis Teague (both 30.7%), and Tayshaun Prince (32.2%). Interestingly, the worst perimeter defenders are dominated by rookies (Doron Lamb, Josh Selby, and Cory Joseph [second year but little playing time]) and lower-tier centers (Viacheslav Kravtsov, Joel Freeland, and Hamed Haddadi).

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