Thursday, August 16, 2007

Defensive Efficiency: Looking at the league leaders

[This post builds off of yesterday's post, so go read that one first if you haven't read it already.]

I had hoped to start off my post today by talking to you about the relative importance of opponent (defensive) turnover rate (DTOR), opponent (defensive) true shooting percentage (DTS%), and defensive rebound rate (DRR) based on the results of the fancy-schmancy regression analysis I was working on (evaluating the impact of those three variables on defensive efficiency). However, when I ran the regression, the coefficients I ended up with struck me as a little bizarre. Since I last took a statistics course four years ago, I've decided that I need to go back and relearn my regression analysis techniques to find out what (if anything) I was doing incorrectly. With that said, I will point out that my R-squared value was 96%, so it seems like the statistics I was looking at do account for most of the variation in defensive efficiency. (In case that sentence wasn't clear: the R-squared value basically tells you how much of the variation in the dependent variable--defensive efficiency in this case--can be explained by the variation in the independent variables tested--DTOR, DTS%, and DRR in this case)

Foiled in my attempt to do fancy statistics, I will just limit myself to discussing what we can learn about what accounts for the success of the league's defensive leaders by eyeballing the data.

Before looking at the data, my initial assumption was that our three key defensive statistics needed to be evaluated together because they are likely to have interactive effects. As an example, a team might play a gambling style of defense. As a result, they might get a number of steals (thus allowing the other team a fewer total number of shots), but when they miss the steal opportunity the resulting shot for the other team might be more wide open (thus resulting in fewer missed shots per shot attempt). In the same way, a team might be so aggressive in contesting shots (forcing more missed shots per shot attempt) that they do not rebound effectively, thus giving the other team more chances for a second shot each possession. For each team, there is probably some optimum trade-off to be made between these different styles of defensive play, but figuring out the optimal trade-off is difficult.

The top 5 teams in the NBA in defensive efficiency in 2007 were the Chicago Bulls, San Antonio Spurs, Cleveland Cavaliers, Houston Rockets, and Detroit Pistons. In looking at the the league-wide ranks of these teams with respect to our three categories, two distinct observations can be made. First, the order of importance of our categories appears to be DTS%, DRR, and then DTOR. Keeping in mind that a "perfect" average ranking in these categories by the top five teams would be a "3" (an average rank of three would mean that all five top defensive teams would be in the top five in that category), the average ranking of these teams was DTS% = 3 (perfect!), DRR = 8, and DTOR = 14 respectively. This order of importance seems to hold true even as we go further down the list of top defensive teams: the average ranking of the top ten defensive teams in these categories was DTS% = 7, DRR = 10, DTOR = 16 (remember, an average rank of "5" would be perfect in this case).

This order of importance also makes sense when you give it some thought. The Golden State Warriors have the best DTOR in the league at about 18%, so even for the Warriors that leaves 82 out of 100 possessions for which DTOR does not help their defensive efficiency. DTS% matters on every single one of these 82 possessions. With DTS% being between 50-57% for every team in the league (the Spurs have the league's best DTS% at 51.06%), DRR matters for about 40 possessions out of every 100. The best defensive teams do the best on the defensive measures that impact the greatest number of possessions.

The second observation is that, while having a good DTS% is obviously key, there is no "correct" trade-off between our three key defensive measures for being a top defensive team. In fact, we can see three distinct paths towards being a top defensive team among our top five. The first path is to be very good (in the top ten) in all three categories. The Chicago Bulls (DTOR = 2, TS% = 3, and DRR = 10) and Cleveland Cavaliers (DTOR = 9, TS% = 5, DRR = 2) reached success going in this direction. I think the key is that both of these teams played fairly mobile big men last season (Ben Wallace and PJ Brown for the Bulls, with an occassional cameo at power forward spot by Andres Nocioni; Drew Gooden and Anderson Verajao for the Cavaliers). Mobile big men can help trap guards (causing more turnovers), while still being able to recover and guard the paint. These big men are also good at rotating quickly for help defense, allowing them to be in position to contest shots and still have good rebounding position. (The Cavaliers also played Zydrunas Ilgauskas, who is not exactly known for being fleet of foot. If the information was readily available, looking at the Cavaliers DTOR with and without Ilgauskas on the floor would be a good way to test my intuition about the role mobile big men play.)

The second path is to be very, very good at DTS% and DRR, but not very good at DTOR. The San Antonio Spurs (DTOR = 20, TS% = 1, DRR = 4) and Houston Rockets (DTOR = 25, TS% = 2, DRR = 1) reached success by following this strategy. For the Rockets, neither Yao Ming nor Dikembe Mutombo was going to successfully trap a guard on the perimeter, but they serve as a strong deterrent in the lane to complement the position defense Jeff Van Gundy has used to great success since his days in New York (it's hard to believe that those Knicks teams were so good on defense despite being so seemingly unathletic). For the Spurs, Tim Duncan is no longer as mobile as he was when he first came into the league, but that sort of big man pressure isn't really the San Antonio way. The Spurs guards chase the shooters off of the 3-point line and into the waiting, shot-blocking arms of a well-positioned Duncan. Solid positioning keeps San Antonio and Houston defenders in place to challenge shots and gobble up the rebounds...and to spots as two of the top defenses in the league.

The third path is to be very good at DTS%, but not particularly good at either DTOR or DRR. The Detroit Pistons (DTOR = 12, TS% = 4, DRR = 24) took this path. In truth, the Pistons are a full step below the other top defensive teams. The Rockets rank fourth in defensive efficiency at 102.18, while the Pistons sit in fifth at 104.68 (for comparison, the Bulls are first at 100.75). More than anything, the Pistons high rank is a testament to the importance of DTS%. Of the top 15 ranked defensive teams, only two of them (the Toronto Raptors, ranked 13, and the Utah Jazz, ranked 15) do not have DTS% within the top 15. The above average mobility of the Pistons big men (Rasheed Wallace and Antonio McDyess) allows them to help cause more turnovers and still recover to contest shots, but they don't rotate and recover quite quickly enough to also grab defensive rebounds effectively.

So what's the moral of the story: Teams can make different trade-offs between our three major defensive categories and still be a top defensive team, but if the trade-offs they make hurt their DTS% then it probably isn't one that will lead to very much defensive success! And without mobile big men, you're unlikely to be able to both force turnovers and defend shots effectively.

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