Aquality control engineer wants to document major problems in a process which produces ball bearings. He measures the di erence D between the nominal diameter of a 5 cm ball bearing and the true bearing diameter. He nds that the bearings are equally likely to have a diameter larger than or smaller than 5 cm. Furthermore, approximately 20% of the bearings have diameters which deviate more than 3 standard deviations from 5 cm. The distribution of D is