Grange, J.T., Baumann, G.W., and Vaezazizi, R. (2003). On-Site Physicians Reduce Ambulance Transports at Mass Gatherings. (Abstract only.) Prehospital Emergency Care. 7(3):322-6.
The authors found that the presence of on-site physicians significantly reduced (p < 0.001) the number of ambulance transports during a large motorsports event in California.