Wednesday, August 15, 2007

More stat talk: Defensive Efficiency

[I know that nothing in this post is particularly new, but I wanted to give an explanation of my ongoing thought process as I figure out what statistics I want to look at when evaluating players.]

If we're going to use statistics to evaluate a player's value to a team, then making sure we're looking at the "right" statistics is obviously the first priority.

Well, what statistics are the right ones? Based on my personal preference, I decided to start my exploration of this issue by looking at the defensive side of the ball. And before you can evaluate an individual players contribution to defensive success, you need to first figure out what factors contribute to a
team's defensive success.

The first question I asked was very straightforward--"how can you tell that a team plays good defense?" The answer was also fairly straightforward (and, I assume, not shocking to anyone reading this blog). You want to look at how many points the other team scores each time they have the ball. Defensive Efficiency (also called points per possession) measures how many points a team gives up per 100 possessions. Using Defensive Efficiency as the measure of defensive ability makes it possible to compare the defensive ability of teams that play at vastly different tempos in a way that using points per game, the most common method for evaluating defenses, does not.

Next, I asked, "How can a team decrease the number of points it gives up per possession?" I think that there are three basic ways that a defense can limit an opponents ability to score per possession: (1) preventing the offensive team from ever getting off a shot, (2) forcing the offensive team to miss a shot, and (3) grabbing the ball after an attempted shot (normally by a rebound) to end the offensive team's possession. If these are the ways a defense can limit an opponent's ability to score, then is it possible to find statistics that measure how a defense performs in each of these three categories? At least at the team level (the level with which we
are currently concerned), the answer appears to be "yes."

A defense can prevent the other team from getting off a shot by forcing a turnover. Types of turnovers include getting a steal, forcing a bad pass (that results in the ball going out of bounds), forcing some other offensive violation (traveling, 3-seconds, moving screen, charge, etc.), forcing a player out of bounds, or a 24 second violation. From the perspective of evaluating a team’s defense, all of these turnovers are equally valuable. Since we want to be able to compare teams that play at different tempos, the important statistic is turnover rate (the percent of an opponent's possessions that result in a turnover).

A defense can force a team to miss a shot in two principal ways. First, the team can play solid position defense and force the opponent to take an off-balance or contested shot or a shot from further away (or a different position) than the offensive player would prefer. Second, the team can block the attempted shot, preventing it from going in the basket. From the standpoint of the team, either method of forcing a missed shot is equivalent. While the most common way to measure this form of defensive performance is to look at field goal percentage defense, this measure is flawed because it does not take into account the difference between 2-point and 3-point field goal attempts. Effective field goal percentage (eFG%) accounts for the different values of these shots, but still is flawed because it does not take into account fouls committed while the other team is shooting or the defensive team is in the penalty or when defensive three seconds is called (all of which lead to free throws, and thus to points for the other team). True shooting percentage (TS%) accounts for free throws, in addition to 2-pointers and 3-pointers, and therefore gives a more accurate indicator of the opposing team's likelihood of scoring when they have a chance to shoot the ball. In its simplest terms, TS% tells you the percentage that the opposing team would have needed to shoot to score its points if they had only attempted 2-point field goals (i.e. no 3-pointers and no foul shots). For a more detailed comparison of TS% to other measures of offensive efficiency, you can read this article by Kevin Pelton.

After an offensive team has attempted a shot that does not go in, a defensive team can end the offensive team's possession by grabbing the basketball. In most cases, grabbing the basketball will be accomplished by getting the rebound of the missed shot. Since we're interested in comparing the defensive efficiency of team's that play at different tempos, the defensive team's ability to obtain a rebound can best be measured by defensive rebound rate, the percent of available rebounds that the defensive team corrals. In a few cases, the defensive team will grab possession of the ball after a shot has been blocked. Grabbing possession of the ball in this way is considered a rebound (I hadn't known this before, but I just received email confirmation on this fact from the webmaster of, so defensive rebound rate accurately reflects a defensive team's full ability to obtain possession of the ball after the other team attempts a shot.

If my thought process is accurate, then we can identify what accounts for a particular team's Defensive Efficiency by looking at the opponent's turnover rate, the opponent's true shooting percentage, and that team's defensive rebound rate. Tomorrow I'll evaluate whether the numbers support my thought process, and I'll also look at what these measurements tell us about what accounts for the defensive efficiency of the NBA's top defensive teams.

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