Why did home-court advantage rise last season?
In the 2017–18 season, home-court advantage spiked in men’s college basketball. In conference games (where the home team is assigned randomly), hosts won 62% of their games and outscored opponents by an average of 3.6 points per game — both major increases over 2016–17, and matching their highest levels this decade:
Of course, the story looks a bit different in longer-term context. Even at 3.6 ppg, HCA last year was lower than it had been for most of college basketball history. And while last season’s jump of nearly a point per game was the largest spike in 20 seasons, it isn’t off the charts in historical terms. (Game-level data comes from sports-reference.com; this research follows up on Ken Pomeroy’s work last year and my research for NYC Buckets).
Still, last year’s increase is notable in light of the secular decline in HCA. Does it mean anything going forward?
Where did home-court advantage rise?
Home-court advantage increased in both major and mid-major leagues last year. The spike was more than twice as large in mid-majors — but those leagues also drove an outsized share of the decrease in 2016–17, so mean-reversion is probably the main reason. (A third group of non-major, multi-bid leagues is not shown due to volatility in the smaller sample, but it also increased significantly in 2017–18.)
It’s perhaps notable that major conferences have very consistently had a larger HCA than mid-majors. One reason might be that major conferences have larger crowds; another reason is that they now generally have to travel farther on road trips, which makes playing away games more challenging. (However, the HCA increase in 2017–18 was visible in road trips of all distances.)
What caused the increase?
In general, crowd size is a plausible driver of home-court advantage, and average NCAA attendance has fallen for 11 straight seasons. That trend continued last year, but the drop (26 fans per game) was less severe than in most seasons. It’s hard to imagine that contributing to an increase in home-court advantage, but even if you think it might, there was no correlation between change in attendance and change in HCA by conference:
What about gameplay factors? Below is the correlation of several nationwide statistical trends (via KenPom) with annual home-court advantage since 1987. Some of these correlations will be spurious — for example, assist rates and HCA have both been declining consistently since 2000, but neither one could reasonably cause the other — but they at least narrow down the candidates of possible drivers.
However, only three of these factors materially changed (+/- 2%) in 2017–18 — and all three of those changes have historically been correlated with lower HCA:
- Free-throw rates fell by 5%
- Three-point attempt rates increased by 3%
- Offensive rebound rates fell by 2%
All in all, the most likely answer is the most boring one: Last year was a fluke. Just as home-court advantage was well below expectations in 2016–17, it came in above expectations in 17–18. Barring a fundamental change in gameplay, attendance, travel, or officiating, there’s no reason to believe HCA won’t fall back to its established trend this year.
Code available here: https://github.com/whitakk/CBB_HCA
