Shafts are cut to length by two machines, A and B. Each machine cuts 50 percent of all shafts in approximately the same amount of time. Machine A's shafts are all good, but Machine B's shafts are all defective. The figure below presents a schematic of machines A and B.

As shafts are finished, they are placed on the conveyor belt and fall into a bin, which can hold 100 shafts. Once a bin is filled, an empty one is placed at the end of the conveyor to take its place. Consequently, on average approximately 50% of the shafts in any bin are from machine A and the other 50% are from machine B.

Bins are taken to the next operation after they are inspected. Data on the number of defectives was collected from 100 percent inspection of every 20th bin. See the data file.

(a) Construct the appropriate control chart.

(b) Interpret the control chart, commenting on its stability, predictability and behaviour.

(c) What next steps would you take? (Note you are not required to actually undertake these steps.