uses an interesting approach:
This paper analyzes influence of an additional referee on number of fouls by using the data from Women's Japan Basketball League (WJBL) in order to examine whether number of police officers affects the crime rate. For the season of 2010-2011, the upper league of the WJBL introduced 3 referees system for the adaption of the international standard. Using this natural experiment, the Difference in Difference and the Instrumental Variable method are used to remove endogeneity. The results indicate that increased number of referees decrease number of fouls after considering both reverse causality and unobservable heterogeneity.
The paper is titled "