The NBA Measures Team Defense Incorrectly

Stephen Dunn

Many advances have been made in NBA stats realm, but despite the advances we still don't measure team defense very well. The impact of a poor offense needs to be removed from defensive metrics in order to have an accurate estimate of defensive ability.

The advancements made in statistics and data analytics in the NBA has been revolutionary. The movement is beginning to leave the traditional box score as a mere relic of the past as terms such as "efficiencies" and "rates" have now become more commonplace. It is no secret that the progress made has been mainly tied to the offensive end, where the individual contribution can be more easily measured. This isn’t to say the defense hasn’t seen any progress; the movement to defensive efficiency is a vast improvement over the old-school metric of opponent’s points per game. However many of the defensive metrics are still quite lacking because it is no easy feat to disentangle the individual contribution to the team’s results. I recognize the immense challenge facing those who try to tackle the measurement of individual defense and thus won’t criticize the current lack of individual defensive metrics. What I do take issue with is our measurement of team defense because the way we are doing it now is quite flawed and the remedy is quite simple.

Before going any further, allow me to change sports for a minute. I doubt too many of this site’s readers follow the Houston Texans on a regular basis, but fans are beginning to burn the jersey of Texans quarterback Matt Schaub. The reason? Schaub has the dubious honor of owning the NFL record for most consecutive games throwing an interception returned for a touchdown (also known as a "pick-six"). Schaub extended his own record last night with yet his 4th consecutive game of throwing a pick-six. While some feel bad for Matt Schaub, I feel worse for the defensive players on the Texans' roster. One look at the stats suggests that, based on points given up per game, the Texans defense is in the bottom third of the league. Unfortunately Schaub’s consistent ability to find the opposing team with his passing has been the direct cause of 20% of the total points given up. The defense had no control over those points, yet anyone looking at points per game will judge them for it. If the impact of Schaub were removed from those metrics the Texans' defense could be a borderline top 10 defense.

It is probably pretty evident where I am going with this piece. The issue I have with how team defense is currently measured is that we don’t adjust the metrics for points scored because of an offense's mistake. Much like a pick-six, a turnover at the top of the key often results in a fast break the other way with few, if any, defenders back to prevent a basket. Despite these points being the result of a poor play on offense, the defense is penalized. If we are to truly judge the defensive capabilities of teams, players, and coaches (schemes) then we should strive to remove this distorting factor.

What is perhaps most frustrating to me is that it isn’t like this information is difficult to track. The NBA already tracks fast break points per game. All that is really needed is the additional tracking of fast break possessions per game. Once that is tracked then it is a straightforward exercise to remove these fast break points and possessions from the total points and possessions given up. This would provide us with the number of points and possessions that the defense was "set" in which we can truly judge the ability of the defense.

Interestingly there is a site that appears to track these defensive metrics: http://www.teamrankings.com. This is the only site I have found that tracks both fast break points and efficiency (from which we can derive possessions). Assuming these stats are accurate, let’s take a look at how these stats would break out for the Lakers last season.

Total Defensive Efficiency: 1.066 points per possession (ranked 20th in the league)

The general consensus is that the Lakers defense was bad last year. Given that defense featured the best defensive player in the league anchoring it (albeit not at 100%), this appears to be a pretty damning indictment of Mike D’Antoni and his lack of defensive focus. This is also the reason that many are claiming the defense has a good shot at being league worst this year since Dwight Howard and Metta World Peace – the only two above-average defenders on the roster – are both gone.

Opponent Fast Break Possessions: 9.1% of total possessions (ranked 28th in the league)

Whether it was turnovers, long rebounds, or just other teams pushing the tempo against a slow-footed roster, teams consistently ran on the Lakers. The opponents "fast breaked" on over 9% of their possessions. The only teams worse were Detroit and Portland.

Opponent Fast Break Efficiency: 1.868 points per fast break (ranked 27th in the league)

Not only did teams fast break often against the Lakers, but they were very efficient when they did. Only three teams gave up more points per fast break: Charlotte, New York, and Sacramento.

Opponent Fast Break Points: 15.9% of total points (ranked 29th in the league)

If teams fast break often and are effective then it is no surprise that teams will generate a large percentage of their points via the fast break. The Lakers were the second worst team in the league with only the lowly Sacramento Kings finishing worse by yielding 16.0% of their points to fast break opportunities.

Given how significant the impact of fast breaks were on the Lakers total defensive metrics, it begs the question of how good or bad their defense was in non-fast break (or "set") possessions.

Defensive Efficiency in Set Defensive Possessions – 0.986 points per possession (ranked 9th in the league)

This is probably the most surprising result when looking at these numbers. Despite what constantly seemed like a porous defense, the Lakers were actually quite stout on that end when they weren’t watching the names on the backs of the opponents' jerseys running down to the other end of the court. Essentially the inclusion of the fast break possessions in the overall defensive efficiency metric pushed the Lakers from a top 10 defense to roughly a bottom 10 defense. No team in the league had a larger change in rankings (11 spots) between the set defensive efficiency and total efficiency.

As shown for the Lakers last year, the fast break component of the overall defensive metrics can be significant. The defensive statistics have evolved from points per game to per possession metrics as a way to remove the skewing effect that pace can have. How much longer must we continue to evaluate defense without removing the skewing effect that fast breaks can have? If we truly want to judge a team's defense, we must look to remove the impact of points resulting from poor offense. Failure to do so will have us assigning blame to the wrong place.

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