This resulted in Wartinger going out and testing the theory. Using a validated, synthetic 3D model of a hollow kidney complete with three kidney stones no larger than 4 millimeters inserted into the replica, he took the model in a backpack on Big Thunder Mountain at the theme park 20 times. His initial results verified patient reports. "In the pilot study, sitting in the last car of the roller coaster showed about a 64 percent passage rate, while sitting in the first few cars only had a 16 percent success rate," Wartinger says.