NBA Home-Court Advantage is in Decline, Are 3s to Blame?

By Brendan Kent

Home courts arenā€™t the fortresses they once were in the NBA. In a piece from 2015, ESPN NBA Insider Tom Haberstroh highlighted a general decline in home winning percentage since 1975, and a sharp decline since the 2011/12 season, with home win percentage falling from 61.2% in 2011/12 to 53.7% in 2014/15.

Haberstroh presents three theories for the decline:

Theory 1: The NBA is more 3-point heavy, less dependent on referees

Theory 2: Technology has allowed teams to handle the road better

Theory 3: Home crowds are no longer the sixth man

Theory 1 is grounded in the idea that home-court advantage stems at least partially from referee bias towards the home team. As Haberstroh notes, ā€œNumerous studies have shown that referees may be involuntarily influenced by the home crowd.ā€ Most fouls are called in the paint, so referees have the most influence when the game is played in the paint. When a team takes a higher percentage of its shots from beyond the arc, referees have fewer opportunities to call fouls and favor the home team.

In determining whether Theory 1 has any clout, we can look at the relationship between a teamā€™s home-court advantage and its 3-point attempt rate. For the purposes of this study, we will define both as such:

Focusing on the 26 seasons since 1989/90, we can visualize the decline home-court advantage:

Further, we can visualize the rise in 3-point attempt rate in the same time frame (the bump in 3-point attempt rate from 1994/95-1996/97 is due to the leagueā€™s temporary shortening of the distance to the arc during that period):

The trends appear to coincideā€”as 3-point attempt rates rose from 1989/90 to 1996/97, home-court advantages decreased, and when 3-point attempt rates leveled off from 1997/98 to 2002/03, so did home-court advantages. 3-point attempt rates have been rising again since the 2003/04 season and so have home-court advantages. Ultimately, however, we need to determine if individual teams with higher 3-point attempt rates have a lower home-court advantages. We test this by regressing home-court advantage on 3-point attempt rate for all teams from 1989/90 to 2015/16. The results are plotted below:

The plot shows what is a statistically significant, negative relationshipā€”that is, teams with a higher 3-point attempt rate tend to have a lower home-court advantage. Via the regression model, as 3-point rate increases by 1 percentage point, home-court advantage decreases by about 0.2 percentage points (i.e. the difference between Home Win % and Overall Win % decreases by about 0.2 percentage points). The p-value on this relationship 2.49e-12.*

The rise of the 3-pointer, it seems, is at least partially responsible for the decline in home-court advantage. Take the game out of the paint, take the influence out of the ref, take some advantage out of the home team.

*[Edited] Note: The relationship remains statistically significant when we control for season

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