Sunday, October 13, 2019

A study on NBA home court advantage

Economist Tyler Cowen often links to NBA studies in his "Marginal Revolution" blog ... here's a recent one, from an August post. (Follow his link to download the study ... you can also find a press release by Googling the title.)

The study used a neural network to try to figure out what factors are most important for home (court) advantage (which I'll call "HCA"). The best fit model used twelve variables: two-point shots made, three-point shots made, and free throws made -- repeated for team at home, opposition on road, team on road, and opposition at home.

The authors write, 

"Networks that include shot attempts, shooting percentage, total points scored, field goals, attendance statistics, elevation and market size as predictors added no improvement in performance. ...

"Contrary to previous work, attendance, elevation and market size were not relevant to understanding home advantage, nor were shot attempts, shooting percentage, overall W-L%, and total points scored."

On reflection, it's not surprising that those other variables don't add anything ... the ones they used, shots made, are enough to actually compute points scored and allowed. Once you have that, what does it matter what the attendance was? If attendance matters at all, it would affect wins through points scored and allowed, not something independent of scoring. And "total points scored" weren't "relevant" because they were redundant, given shots made.


The study then proceeds to a "sensitivity analysis," where they increase the various factors, separately, to see what happens to HCA. It turns out that when you increase two-point shots made by 10 percent, you get three to four times the impact on HCA compared to when you increase three-point shots made by the same 10 percent.

The authors write,

"[This] suggests teams can maximize their advantage -- and hence their odds of winning -- by employing different shot selection strategies when home versus away. When playing at home, teams can maximize their advantage by shooting more 2P and forcing opponents to take more 2P shots. When playing away, teams can minimize an opponent's home advantage by shooting more 3P and forcing opponents to take more 3P shots."

Well, yes, but, at the same time, no. 

The reason increasing 2P by 10 percent leads to a bigger effect than increasing 3P by 10 percent is ... that 10 percent of 2P is a lot more points! Eyeball the graph of "late era" seasons the authors used (I assume it's the sixteen seasons ending with 2015-16). Per team-season, it looks like the average is maybe 2500 two-point shots made, but only 500 three-point shots.

Adding 10 percent more 2P is 250 shots for 500 points. Adding 10 percent more 3P is 50 shots for 150 points. 500 divided by 150 gives a factor of three-and-a-third -- almost exactly what the paper shows!

I'd argue that what the study discovered is that points seem to affect HCA and winning percentage equally, regardless of how they are scored. 


Even so, the argument in the paper doesn't work. By the authors' own choice of variables, HCA is increased by *making* 2P shots, not my *taking* 2P shots. Rephrasing the above quote, what the study really shows is,

"When playing at home, teams can maximize their advantage by concentrating on *making* more 2P and on forcing opponents to *miss* more 2P. That's assuming that it's just as easy to impact 2P percentages by 10 percent than to impact 3P percentages by 10 percent."

But we could have figured that out easily, just by noticing that 10 percent of 2P is more points than 10 percent of 3P.


The authors found that you increase your HCA more with a 10 percent increase in road three-pointers than by a 10 percent increase in road two-pointers. 

Sure. But that's because, with the 3P, you actually wind up scoring fewer road points. Which means you win fewer road games. Which makes your HCA larger, since winning fewer road games increases the difference between home and road. 

It's because the worse you do on the road, the bigger your home court advantage!

Needless to say, you don't really want to increase your HCA by tanking road games. The authors didn't notice that's what they were suggesting.

I think the issue is that the paper assumes that increasing your HCA is always a good thing. It's not. It's actually neutral. The object isn't to increase or decrease your HCA. It's  to *win more games*. You can do that by winning more games at home, increasing your home court advantage, or by winning more games on the road, decreasing your home court advantage.

It's one of those word biases we all have if we don't think too hard. "Increasing your advantage" sounds like something we should strive for. The problem is, in this context, the home "advantage" is relative to *your own performance* on the road. So it really isn't an "advantage," in the sense of something that makes you more likely to beat the other team. 

In fact, if you rotate "Home Court Advantage" 360 degrees and call it "Road Court Disadvantage," now it feels like you want to *decrease* it -- even though it's exactly the same number!

But HCA isn't something you should want to increase or decrease for its own sake. It's just a description of how your wins are distributed.

Labels: , ,