# Understanding the NBA: Explaining Advanced Defensive Stats and Metrics

Adam Fromal@fromal09National NBA Featured ColumnistJanuary 27, 2012

# Understanding the NBA: Explaining Advanced Defensive Stats and Metrics

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This is the second part of my three-part series about understanding the complexities of NBA statistics. The next few slides deal with advanced defensive statistics.

While there's no such thing as a perfect statistic, there is such a thing as the perfect application of a statistic.

While some people believe too fully that there is nothing more to basketball than the "human aspect," others realize that numbers can explain a whole lot about the action on the court. Those that embrace the numbers generally tend to see their knowledge of the game increase, but only if they know how to use the stats.

I do understand that some of the advanced stats and metrics can be a bit intimidating because they're unfamiliar and, in some cases, complicated. It doesn't help that we're force-fed traditional, sometimes useless stats by the media.

Every time I see someone immediately dismiss an advanced statistic because it's "useless," it kills a little part of me. No stat is worthless if you know how to use it.

The following five slides explain the most common of the advanced defensive stats and metrics. Click here for my explanation of the offensive statistics. Check back later for the overall ones.

And as always, feel free to leave any comments or questions in the appropriate section.

You can find all of these stats and more at Basketball-Reference.com82Games.com and HoopData.com.

# Steal Percentage (STL%)

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Calculation

100*(S*(TMP/5))/(MP*OP)

Where S=Steals, TMP=Team Minutes Played, MP=Minutes Played, OP=Opponents' Possessions

Explanation

While having a player steal the ball is useful, having a player steal a higher percentage of the time is even more useful. Put quite simply, wouldn't you rather have a point guard rack up five steals in 40 minutes played instead of a point guard take 120 minutes of action to garner the same five steals?

Steal percentage takes this preference into account as it calculates the percentage of an opponent's possessions that end up with the player in question swiping the ball away and starting the ensuing offensive possession, provided that he's on the court for the possession.

This is one of the stats that we like to call "tempo-free," meaning that it's a stat adjusted for pace and volume.

A player going up against a fast-paced team like the Miami Heat or Washington Wizards is going to have more defensive opportunities to rack up counting stats, leading to elevated numbers of steals per game and steals per minute (which is also a better way of looking at steals than just steals by themselves).

Similarly, a player who plays 35 minutes per game is going to have more opportunities to put up impressive steals numbers than a player on the court for 20 minutes per game. It may seem like common sense that averaging two steals per game in 20 minutes of action per contest is more impressive than averaging two steals per game in 35 minutes of action per contest, but that distinction is lost when only steals per game is cited.

Because steal percentage is free from the effects of pace and volume, it's a better indication of how effective a player is at racking up the thefts during each and every one of the team's possessions.

Limitations

Without looking at play-by-play data, the best this stat can do is provide an estimate of the aforementioned percentage it is meant to calculate.

Additionally, there is no way to track anything other than successful gambles resulting in steals. An often overlooked aspect of defensive play is the unsuccessful ventures at thievery that result in an opponent blowing by the defensive player.

Example Usage

"Monta Elllis' STL% of 2.5 percent ties him for 19th in the NBA this season."

How I interpret that sentence: Monta Ellis' STL% makes him look like a better defender than he actually is because it doesn't account for the many unsuccessful steal attempts that the Golden State Warriors guard makes.

# Block Percentage (BLK%)

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Calculation

100*(B*(TMP/5))/(MP*(OFGA-O3PA))

Where B=Blocks, TMP=Team Minutes Played, MP=Minutes Played, OFGA=Opponent Field-Goal Attempts, O3PA=Opponent Three-Point Attempts

Explanation

While having a player block a shot is useful, having a player block shots a higher percentage of the time is even more useful. Put quite simply, wouldn't you rather have a center rack up five blocks in 40 minutes played instead of taking 120 minutes of action to garner the same five blocks?

Block percentage takes this preference into account as it calculates the percentage of an opponent's two-point attempts that end up with the player in question blocking a shot.

This is one of the stats that we like to call "tempo-free," meaning that it's a stat adjusted for pace and volume.

A player going up against a fast-paced team like the Miami Heat or Washington Wizards is going to have more defensive opportunities to rack up counting stats, leading to elevated numbers of blocks per game and blocks per minute (which is also a better way of looking at blocks than just blocks by themselves).

Similarly, a player who plays 35 minutes per game is going to have more opportunities to put up impressive blocks numbers than a player on the court for 20 minutes per game. It may seem like common sense that averaging two blocks per game in 20 minutes of action per contest is more impressive than averaging two blocks per game in 35 minutes of action per contest, but that distinction is lost when only blocks per game is cited.

Because block percentage is free from the effects of pace and volume, it's a better indication of how effective a player is at racking up the rejections during each and every one of the team's possessions.

If that explanation sounded similar (or almost identical to the one provided for steal percentage), it's because it was. The two evaluations of performance do almost the same thing, just for different basic defensive stats.

Limitations

Without looking at play-by-play data, the best this stat can do is provide an estimate of the aforementioned percentage it is meant to calculate.

The other problem is that it only takes two-point shots into account in the denominator while allowing for the possibility of blocked three-pointers in the numerator. The formula might have to assume otherwise because of the rarity of blocked three-pointers, but a successful block from outside the arc would add one to the blocked-shot total without a shot attempt being technically recorded in the formula.

This makes the stat slightly skewed in the positive direction.

Example Usage

"Bismack Biyombo has the best BLK% in the NBA right now with a mark of 9.2 percent, well ahead of JaVale McGee's second-place 7.9 percent."

How I interpret that sentence: One of the indications that Biyombo, a rookie for the Charlotte Bobcats, is going to become a productive player is that he's insanely efficient when it comes to blocking shots.

# Defensive Rebounding Percentage (DRB%)

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Calculation

100*(DRB*(TMP/5)/(MP*(TORB+ODRB))

Where DRB=Defensive Rebounds, TMP=Team Minutes Played, MP=Minutes Played, TORB=Team Offensive Rebounds, ODRB=Opponents' Defensive Rebounds

Explanation

Whenever a shot clanks off the rim, there are four possible outcomes: The ball could go out of bounds and be counted as a defensive team rebound; the ball could bounce off a defensive player and go out of bounds to be counted as an offensive team rebound; the ball could be pulled down by a defensive player and counted as a defensive rebound; or the ball could be pulled down by an offensive player and counted as an offensive rebound.

If you add up all four of those results, you account for all of the potential rebounds in a game.

Defensive rebound percentage calculates the percentage of available defensive rebounds that a player grabs while he's on the court.

This is the last of the commonly-used defensive tempo-free rate stats and much like the previous two, it doesn't account for pace or volume.

Limitations

Once more, it's too much trouble to go back and retroactively look at all historical box scores. No one really wants to do that.

Therefore, this is merely an estimate, albeit an accurate one.

Example Usage

"Dwight Howard has the best DRB% in the league at 24.0 percent and it's really not even close."

How I interpret that sentence: Don't bother going up against D-12 if he's on the defensive glass and you're playing on the team that just shot the ball.

# Defensive Rating (DRtg)

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Calculation

Way too complicated to explain here. Full calculation is provided in Dean Oliver's book Basketball on Paper.

Explanation

Defensive rating simply shows how many points a player allows per 100 possessions.

This statistic works differently than a simple plus/minus system, where all points scored while a player is on the court count against him. Only the buckets that are scored as a result of his defensive lapses are counted against him.

Defensive rating also eliminates factors like pace of play and minutes played per game.

It is also important to note that as opposed to most other basketball stats, lower numbers are better when it comes to defensive rating.

Limitations

The one blindingly obvious limitation of defensive rating is our dependency on sites like Basketball-Reference.com.

Because of the estimations made in Oliver's formula, this is once more just a baseline and not an exact number.

Example Usage

"You might be quite surprised to hear that Omer Asik has the best defensive rating (90.4) in the NBA."

How I interpret that sentence: Omer Asik has played tremendous defense off the bench, as his 90.4 DRtg indicates, but he's probably helped by the fact that he's not playing too many minutes per game.

# Defensive Win Shares (DWS)

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Calculation

(MP/TMP*TDP*(1.08*LPPP-DRtg/100)/(0.32*LPPG*(TP/LP))

Where MP=Minutes Played, TMP=Team Minutes Played, TDP=Team Defensive Possession, LPPP=League Points Per Possession, DRtg=Defensive Rating, LPPG=League Points Per Game, TP=Team Pace, LP=League Pace

Explanation

If this calculation seems intimidating right now, imagine how intimidating it would be if Dean Oliver's couple formula for Defensive Rating was included in it.

The formula's numerator is essentially a calculation of a player's marginal defensive value, or the quantifiable amount that the team's defensive ability increases while the player is on the court.

As for the denominator, it's simply the marginal points per win. In a less jargon-y sentence, the denominator uses a team's pace of play to adjust the player's contributions to a more standard rate.

Defensive Win Shares takes all facets of the defensive game into account and is tempo-free.

The metric is scaled so that one DWS is equal in value to one win added to the player's team's record.

Limitations

Compared to the rest of the stats included in this slideshow, DWS doesn't have too many flaws. The only true flaw is that while it accounts for pace of play, it's still a counting stat of sorts.

Although different from counting stats like total blocks, there is still a clear benefit to players who are on the court for a lot of minutes.

Moreover, we have to rely on databases to calculate the stat for us as DRtg helps to make up the formula.

Example Usage

"Josh Smith really is an elite defender according to his 1.4 DWS."

How I interpret that sentence: Because J-Smoove both plays a lot of minutes and plays efficiently on the defensive end of the court at all times, he's near the top of the league leaderboard when it comes to DWS.

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