The supervisor in Example 8.1 is told by the plant owner that the thickness of the copper plating must not be less than .025 millimeter in order for the process to be in control. To decide whether or not the process is in control, the supervisor might devise a test. He could hypothesize (or assume) that the process is in control—that is, assume that the average thickness of the copper plating is .025 or greater—and use samples from several days of operation to decide whether or not his hypothesis is correct. The supervisor’s decision-making approach is called a test of hypothesis.