Continuing on from question #4, the employee continues to track the same data for 10 weeks after the supplier change, and gets the following data:

Sample data:	1	2	3	4	5	6	7	8	9	10
Mean	27.8	28.9	27.4	28.7	28.0	27.1	27.2	27.3	28.0	27.4
Range	2.3	1.5	3.0	2.1	3.2	2.2	3.6	2.0	3.4	1.7

The Operations manager looks at the data and points out that the strength has gone down as he claims, as the average of these new readings is below the previous average, and the average range of the samples is also higher, which he claims proves there is more variability. Should the company change back to the original supplier? Why or why not?

• All the new sample means fall within control limits, as do all the new sample ranges. Therefore, the process is under control, which means there has been no assignable variation. The supplier change therefore did not have an impact and they should not change back.

• The operations manager is correct, and there the new supplier has added additional variability to the process. They should therefore change back to the original supplier.

• Calculating new control limits for the new data, shows that the process is in control, so they should not change back.

• Calculating new control limits for the new data gives clearly different control limits. There therefore has been an assignable change in variation, so they should change back to the original supplier.