Estimating the effect of attendance on home advantage in the National Basketball Association

Using COVID-19 safety protocols as a natural experiment, we are able to delineate three distinct attendance categories in the NBA: 1) unrestricted games played prior to the pandemic, 2) attendance-restricted games played with socially distanced fans, and 3) 'ghost games' played without fans. Further, since attendance at restricted games was exogenously determined by local COVID-19 protocols that were in turn driven by changes in COVID-19 case counts, we are able to estimate whether the 'marginal fan' contributes to home advantage. Taken together, our results indicate that the presence of fans matters to home team performance; in fact, 'ghost games' eliminated home advantage in totality. With a relatively small number of socially distanced fans, however, the entirety of home advantage was retained. Interestingly, since the size of socially distanced crowds had a statistically insignificant impact on home advantage, we find no evidence of a 'marginal fan' effect. Finally, since researchers have found that officiating is influenced by fans in international soccer (e.g. Anders and Rotthoff, 2014), we explore whether NBA officiating behaviour was altered due to changes in attendance conditions. Our results indicate that NBA officials were not measurably influenced by the presence or quantity of fans.