NBA Defence And Regression To The Mean In 2013-2014


What can we take from last season's NBA 3 point defence shooting stats and what impact does a team's interior defence have on opponents shooting from outside? Today Ed Feng explains.

Chicago Bulls Stadium

The 3 point shot can change a basketball game. When a shooter like Steph Curry gets hot and goes 11-13 in Madison Square Garden, it not only gets fans on their feet but allows lesser teams to slay Goliath (although Golden State still lost to New York).

As the smart bettor, you know 3 pointers make a large contribution to the variance in margin of victory. But does 3 point shooting have anything to say about preseason NBA predictions?

Yes, 3 pointers can impact a team's outlook for the season. Let me explain.

Regression in 3 point defence

A NBA team's 3 point field goal defence regresses to the mean each season. Over the last 10 seasons, 3 point field goal defence percentage one season explains only 2.7% of the variance in 3 point field goal defence the next season (correlation coefficient of 0.165). It regresses as strongly to the mean as free throw defence.

However, 3 point defence isn't all random. Stingy defences that allow lower field goal percentage on 2 pointers also defend the 3 point line better. Over the last 10 seasons, 2 point field goal defence explains 20% of the variance in 3 point field goal defence (correlation coefficient of 0.45). Closing out shooters at the arc is a skill. For example, Indiana led the NBA in both 2 and 3 point field goal defence in 2012-2013.

To find teams that had good or bad luck from opponent's 3 point shooting, we look for teams with a strong mismatch in 2 and 3 point field goal defence last season. Below, we discuss how the Los Angeles Clippers and Atlanta Hawks had a strong defence within the arc but gave up a high rate of 3 pointers. They were unlucky. The Portland Trailblazers had the opposite tendencies. This has consequences for the up coming season.

Los Angeles Clippers

The Clippers have emerged as the most exciting team in Los Angeles if not the entire NBA. They feature the best player not named LeBron (Chris Paul) and a human highlight reel (Blake Griffin). Last season, the Clippers finished 4th in the West with a 56-26 record. Can they rise further and contend for a title?

Basketball analytics says yes. The Clippers played great defence last season, ranking 6th in field goal defence in the NBA. However, opponents shot 37.3% from behind the arc, 26th worst in the league and significantly above the league average of 35.9%.

If opponents had shot the league average on 3 pointers, the Clippers would have allowed 72 fewer points over the season. This corresponds to 2.5 more wins, which would have pushed them past Denver and San Antonio to 2nd in the West. Odds say they would have beat the Lakers in the first round. Instead, the Clippers lost to Memphis.

If 3 point defence regresses to the mean, the Clippers should do well this season. In the off season, they added Jared Dudley, a forward that has shot 40.5% on 3 pointers for his career. They also picked up Byron Mullens, a seven footer that took a crazy 37% of his shots from 3 point range last season.

Atlanta Hawks

While the Hawks went 44-38 and placed 6th in the East last season, they lost do it all big man Josh Smith to free agency during the off season. Moreover, they changed coaches by hiring Mike Budenholzer, a former assistant for San Antonio. Can the Hawks stay relevant in the Eastern Conference playoff picture?

Better luck on three pointers will help. The Hawks were a good defensive team last season, ranking 9th in 2 point field goal defence. However, they finished 29th in 3 point defence, as opponents shot almost 38% from behind the arc.

If Atlanta's opponents shot the league average last season, they would have allowed 99 few points. This equates to 3.4 more wins. While this only bumps them from 6th to 5th in the East, they would have avoided Indiana in the first round. Indiana beat Atlanta in their playoff series and gave eventual champions Miami all they could handle in the Eastern Conference Finals.

Even though Smith departed, the remaining core of Al Horford, Jeff Teague and Kyle Korver remain in place for the Hawks. In addition, they added forward Paul Milsap, a player ESPN Magazine ranks as 7th best in the NBA according to adjusted plus minus. With some better luck in 3 point defence, the Hawks will make the playoffs again.

Portland Trailblazers

The Trailblazers were in the Western Conference playoff picture for most of the season last year. Despite their decline towards the end of the season, the team looks promising. Point guard Damian Lillard won Rookie of the Year, and French wing player Nicholas Batum and post scorer LeMarcus Aldridge round out the nucleus of this team. Can they contend for playoff spot in the tough Western Conference?


Portland had a bad defence last season. They allowed opponents to shoot 51.2% on two pointers, 29th out of 30 teams. However, their opponents magically started clanking jumpers once they stepped outside the arc. The Trailblazer's opponents shot 34% from 3 point range, 3rd worst in the NBA.

Had opponents shot the league average from 3 point range, the Trail Blazers would have allowed 87 more points. This corresponds to losing 3 more games for a team that went 33-49.

Portland upgraded this off season by adding defensive minded big Robin Lopez and dumping an ineffective JJ Hickson. Moreover, coach Terry Stotts has his team on the cutting edge of analytics. However, their luck in 3 point defence last season probably won't persist, making their climb to a playoff spot in the West more unlikely.

Rankings for 3 point defence

Basketball Reference is a great resource for NBA statistics. Their 2012-2013 league page allows you to sort by both 2 and 3 point field goal defense. See if the randomness of 3 point defence might affect your team.

If you're interested in further reading on this topic, Ken Pomeroy offers some interesting analysis of college basketball, looking at the randomness of 3 point shooting and how interior defence impacts 3 point shooting.



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Tags:  NBA , Betting Analysis

Im the founder of The Power Rank, a sports analytics website. Its based on a ranking algorithm I developed from my Ph.D. thesis at Stanford. I freelance with Betting Expert and Sports Illustrated.