Although it didn’t seem assured at every point this season, the Celtics will absolutely be making a playoff appearance, taking on a familiar foe in the Atlanta Hawks. This weekend they will arrive in the postseason, riding a crest of incredible defensive performances. Since April 1st they’ve allowed just 95.5 points per 100 possessions, the 3rd best mark in the NBA over that stretch, and a number that would lead the league if it had been carried through since December.
While it has jumped a level in the past few weeks, the Celtics’ defense has been good all season, built primarily on two pillars. They force a lot of turnovers – on 14.6% of their opponent’s possessions, the 5th best mark in the league. They also force a lot of misses. On the season the Celtics have allowed their opponents an eFG% of 45.2%, lowest in the league this season and the 7th lowest mark in the NBA over the last ten years. Defense, and eFG% allowed, has been a staple of the Celtics since the Voltron-esque assembly of Ray Allen, Paul Pierce and Kevin Garnett, but this season has been their most effective effort.
That mark of 45.2% speaks volumes to what the Celtics’ have accomplished at the defensive end, but it also conceals some truths. Their opponents are actually shooting a respectable 44.8% on two-pointers. Their eFG% mark is inflated, or deflated as the case may be, by the fact that Celtics’ opponents are shooting just 30.4% on three-pointers this season. If that mark holds over the rest of the season it would be one of the lowest in the NBA over the last twenty years.
Measuring three-point defense is a curious thing. If we assume that every shot, no matter how ludicrous and ill-advised, has at least an infinitesimal chance of going in, than the most effective means of defense would be to keep your opponent from taking three-point shots. However, players like Jordan Crawford and Kobe Bryant blow a hole through this measurement slant with their dogged determination to continue taking three-pointers irregardless of the defense, distance from the basket, time on the clock and game situation. But just looking at the flip side of the coin, shooting percentages, does not cover the whole defensive picture either, because of some player’s tendencies to help the defense by taking bad shots, and other player’s tendencies to wait for open shots.
Ken Pomeroy has done some work with defensive 3PT% over the past few months and his conclusions may surprise people. Pomeroy works with college basketball, but his evaluation was that opponent’s 3PT% is a defense independent statistic, that defense essentially has no impact on their opponent’s 3PT%. What they can exert some measure of control over is how many attempts their opponents get. Pomeroy’s analysis is difficult to replicate with the NBA because no portion of an NBA team’s schedule represents an exact duplication of a previous portion, like you find in college basketball during conference play. Still, Pomeroy’s assertion that three-point attempts allowed is a defensive statistic of tremendous import makes sense in the NBA arena.
Assuming that a team’s true ability to keep their opponents from scoring on three-pointers is reflected in both limiting makes and limiting attempts, I tried to devise a method to examine three-point defense as a combination of both factors.
The graph below is a scatter plot, with each point representing one season of Boston’s defense against one NBA team, for example, vs. Atlanta, 2009-2010. The X-axis represents the change in the opponent’s three-point attempts per game when they played Boston, against their season average. The Y-axis represents the change in the opponent’s three-point percentage when they played Boston, against their season average. The blue points are team/season defensive performances from 2007-2008 through the 2010-2011 season. The red points are from this season.
The graph is divided up into quadrants so that each point can be quickly categorized. For example, every point in Quadrant B represents a season-long series in which the Celtics held a team to fewer three-point attempts per game than they usually take AND a lower percentage than their season average.
A point in Quadrant B, fewer attempts AND makes for your opponent, would seem to be the ultimate goal here. Obviously, any point in Quadrants A or D would represent a team that, throughout a season series with Boston, shot better than their season average; not a desirable defensive outcome.
I can see the appeal of Quadrant C – if a team isn’t making a high percentage it seems like it would be advantageous to have them shoot as often as possible. But to me this piece seems like one that is out of your control. You lower the shooting percentage of your opponents by presenting them with difficult shooting opportunities, the same way you would theoretically reduce their attempts. If your opponent ignores one half of that equation and continues to shoot, it’s a bonus, but not necessarily something that you can encourage while simultaneously holding the line on percentage. It’s not very plausible that an opponent could be presented with a shot that would appear to be an open high-percentage look, which then becomes highly contested between the player making the decision to shoot and the ball leaving their hand. If you aren’t playing the Wizards every night and don’t have the luxury of counting on your opponent to shoot themselves in the foot with shot selection, than Quadrant B seems like the ideal.
Interestingly enough 35 teams over the past five season have fallen into Quadrant C against the Celtics. 16 of those 35 points come from the 2007-2008 season, and another 9 come from this season. Quadrant C would seem to represent the results most heavily influenced by luck, and it’s no surprise that those results show up most frequently in the Celtics’ two best overall defensive seasons of the Garnett-Pierce-Allen era. That’s not to say that the Celtics’ defense hasn’t been good just that it has also, like every team but the Bobcats, at times benefited from being lucky.
There is a faint whiff of random good fortune, but we also see some patterns here, revealing the influence of intention. Since 2007-2008 the Celtics were able to hold 63.4% of their opponents below their season-long three-point percentage, across a season series. They were able to hold 60.0% of those opponents to fewer three-point attempts than their season average. This season the Celtics are holding 79.0% of their opponents to below their season-long three-point percentage and 62.0% of their opponents to fewer three-point attempts than their season average. Although the peaks of the past five years may be established by luck, the foothills have clearly been built by the Celtics’ defensive system. So what exactly are the Celtics doing to keep their opponents from scoring on the perimeter?
Breaking down X’s and O’s is not my forte, but I gave it a stab as a part of this work. I spent hours watching opponents take three-pointers against the Celtics this season, and still can’t point to any definable pattern. The Celtics challenge shots, but certainly not aggressively or consistently enough to attribute the difference to mere effort. I couldn’t find anything repetitive in the way they double-team the post, funnel penetration, or defend pick-and-rolls that would explain the difference. either There’s something there, but it may take more a more keen observer than me to find it.
Looking ahead to a matchup with the Hawks, this ability to defend the three-point line should be a huge asset. The Hawks are 11th in the league in three-point attempts per game and 5th in the league in three-point percentage. They rely heavily on those long-range shots to power their offense – on the season 23.0% of their scoring has come on three-pointers, the 8th highest total in the NBA. In their three matchups this season the Celtics have held the Hawks to an ORtg. of just 91.1. They’ve kept the Hawks efficiency down by taking away this weapon, reducing their three-point attempts by 3.2 per game, and their percentage by 6.1%. Against the Celtics, the Hawks have scored just 18.5% of their points on three-pointers. This series presents a chance for the Celtics to test their defense against a team that relies heavily on the exact shot they excel at taking away.
Even if the source of this defensive strength can’t be pinpointed exactly, its presence is undeniable. The old phrase “It’s better to be lucky than good” has weight. The Boston Celtics seem to have figured out how to achieve both, and have put themselves in a great position to do some damage in the playoffs.
Statistical Support For This Story From NBA.com