The degree to which nuts and bolts are tightened in numerous places on a car is often important. For example, in Toyota cars, a nut holds the rear signal light. If the nut is not tightened sufficiently, it will loosen and fall off; if it is too tight, the light may break. The nut is tightened with a torque wrench with a set clutch. The target torque is 8 kgf/cm (kilogram- force per centimeter) with specification limits LSL = 7kgf/cm and USL = 9 kgf/cm. Statistical process control is employed to constantly check the process. Random samples of size 4 are drawn after every 200 nuts are tightened. The data from the last 25 samples were recorded.

a. Determine whether the process is under control.

b. If it is out of control, identify when this occurred and the likely cause.