Statistical Analysis. Humor. Knicks.

Wednesday, September 17, 2014

Home Is Where the Background Is?

I have various saved incomplete blog entries that will never see the light of day. One of them was about what gives a team the home court advantage. Last week, Raptorblog asked the same question:

In my mind, the greatest mystery about NBA basketball is why homecourt advantage has such a profound effect on game results. I understand that the home team is allowed final substitutions and has the support of their fans (outside of Atlanta and New Orleans) but I can’t figure out why NBA home teams have a higher winning percentage than the other three major sports.

So I decided to revive one of my unfinished posts.

Everyone reading this (I hope) is familiar with the scene from Hoosiers where the team makes the finals & they go to visit the court they will be playing on. Gene Hackman (the coach) has the players measure various parts of the court. The hoop is exactly 10 feet high, just as their home court. This is to prove to his players that this (and all) courts are exactly the same dimensions (unlike baseball stadiums, or even football fields with their different turfs & weather). So in basketball we can eliminate any kind of home field bias due to the playing surface (although some people claim that the floors of some courts have weird bounces, I think everyone can agree that this is highly unlikely of a 61% home field advantage).

To figure out what gives a team the home court advantage, I decided to take the playoff teams and split them into 2 groups, the top 8 & bottom 8. I choose these groups to isolate a few variables. First the bottom 8 teams all lost & were vastly inferior to their opponents. Second they played less games than the top 8. Using ESPN I was able to get their home & road splits. I also decided to use the regular season statistics as well, getting the home/road splits for every team this year. So what kind of theories do we have?

Theory #1: Home Cooked Refs

Referees with their fragile egos & fearful of being booed give the home team better foul calls. If this is true teams will have less free throw attempts on the road than at home. So what do the numbers say?

[Note: the first set of numbers are the home numbers, the second set road numbers, the third set the difference.]

TEAM	FTM	FTA	FT%	FTM	FTA	FT%	FTM	FTA	FT%	

Top 8 18.9 26.7 70% 19.1 26.5 72% -0.2 0.2 -2%
Bot 8 16.2 22.3 73% 18.1 23.3 78% -1.9 -1.0 -4%

There really isn’t much of a difference in shots attempted. On the road the bottom 8 playoff teams averaged an extra of 1 free throw per game with a higher percentage, so if anything it appears that the bias is the other way. In fact just to be sure I checked with the regular season stats. Teams attempted only 1.2 more free throws at home than on the road. The top 5 teams in getting more FTA at home were: Atlanta, Golden State, New Orleans, Memphis & Milwaukee; meanwhile the bottom 5 teams were a mixed bag as well: Philly, Washington, Boston, Houston, and Toronto.

I really can’t conclude anything from this, but I would certainly lean to the side that refs don’t give the home team special treatment with respect to free throws. Having one extra free throw per game doesn’t seem to be a large advantage.

Theory #2: Better Free Throw Shooting

Maybe the refs don’t give players an advantage, but once the player arrives at the charity stripe do the fans make the difference? When a home player is shooting the fans are calm, but when an opposing player is trying to make a free throw the fans go nuts, trying to distract him from making his shot. What do the numbers say?

Using the same chart as above, oddly enough teams in the playoffs this year have shot better on the road. Again to verify my results I’ll look at the regular season. The league shot a FT% of 75.2% at the comfort of home this year, and 75.3% on the road. That’s right on the road they shot 0.01% better. Certainly not significant.

Theory #3: Able to See the Rim Better

On the Raptorblog.com comments for his post, Kamahsutra came up with this theory:


It has been noted for basketball, that the home field advantage may be due to the shooting. No doubt all the court dimensions are exactly the same in all NBA arenas, however the rest of the surroundings are not, eg, the look of the backboard relative to the background. Anything that may affect the shooter’s comfort level. This would be tell tale, if you were able to look at the shooting percentages of visiting against home teams during playoffs.

I tend to notice this at my local gym. The guys that have been playing there for years have little adjustment period when the season starts up. However when I bring someone new there, they always seem to struggle to hit their shots. It’s a large size court – something that’s rare to find in New York city parks. The backboard is glass (again rare to find in public parks), and the background is a monotonous beige. At times it’s hard to pickup the rim, but since I’ve been there for 4 years, now I have no problem knowing where it is. Once or twice we’ve had to use another gym for a short period of time, and my first time there I have trouble hitting my shots.

Could this be true in the NBA?

TEAM	FGM	FGA	FG%	FGM	FGA	FG%	FGM	FGA	FG%	
Top 8 35.0 76.7 45.6% 31.7 76.6 41.5% 3.3 0.1 4.2%
Bot 8 34.6 81.8 42.5% 30.6 77.7 39.3% 4.0 4.1 3.2%

The initial numbers look good. The good teams shot 4% better at home, and the bad teams were 3% better. The regular season shows similar yet reduced results: 44.5% at home, 43.2% on the road. The reason for the regular season numbers being smaller is the better teams make the playoffs. For example sorting the regular season by biggest difference in FG% would put most of the playoff teams near the top (Dallas, San Antonio, Milwaukee, New Jersey, Miami, and Sacramento), and weaker teams at the bottom (Chicago, Indiana, Boston, Cleveland, Seattle, and Orlando).

What if this is due to another reason. One could be maybe teams on the road feel more desperate and shoot more 3s (which would lower their fg%). Let’s try to break down FG% & see where the difference is, first with two pointers:

TEAM	2PM	2PA	2P%	2PM	2PA	2P%	2PM	2PA	2P%	
Top 8 29.2 60.7 48.2% 26.7 61.2 43.7% 2.5 -0.5 4.5%
Bot 8 29.3 65.9 44.9% 26.2 63.1 41.3% 3.1 2.8 3.6%

There seems to be a significant difference here, around 4%. The regular season nets a smaller 1.2% difference. There are 10 of the 16 playoff teams which rank in the top half. Now about FG%’s other half, three pointers?

TEAM	3PM	3PA	3P%	3PM	3PA	3P%	3PM	3PA	3P%	

Top 8 5.8 16.0 35.9% 5.0 15.4 32.1% 0.7 0.6 3.8%
Bot 8 5.2 15.9 33.3% 4.4 14.6 29.3% 0.9 1.3 4.0%

Again a big difference here, near 4%. During the regular season, the home team had a 1.3% advantage at home when shooting threes. So to conclude this section teams in the playoffs this year so far have shot about 4% better overall which is about evenly distributed between 2 pointers & 3 pointers.

Right now I have to conclude that this is a possibility. The cause may not be what we think, but the results are clear. There could be a host of reasons why teams shoot better at home, including sleeping in your own bed or eating familiar food. However I don’t think science has come far enough that we can isolate such variables.

Is this enough evidence to come to a certain conclusion that it’s the backdrop that effects shooters? No. But it’s certainly a start.