You have just installed a new system that uses an interferometer to measure the thickness of polystyrene film. To control the thickness, you plan to measure three film specimens every 10 minutes and keep and s charts. To establish control you measure 22 samples of three films each at 10-minute intervals. Table 12.11 gives and s for these samples. The units are millimeters . Exercises 12.55 through 12.57 are based on this process improvement setting.