Question

Answer the following questions:

1. What is the difference between common cause variation and special cause variation?

2. What should you do to improve a process when special causes of variation are present?

3. A company requires a C_pk value of 1 or larger. If a process has C_p = 1.5 and C_pk = 0.8, what changes should you make to the process?

4. Why is a capability analysis not performed on out-of-control processes?

Answer 1

1. Common cause patterns are predictable, quantifiable and usually appear in a process where special cause patterns are unpredictable,unquantifiable and unusually presented in a process.
2. The process can be improved by changing the component or the system process in a better way.
3. If he Cpk is greater than zero and less than one, then some part of the process output is outside the specification limit. Hence the company should identify the outliers which are outside the limit and rectify accordingly.
4. All methods of capability analysis is done on a statistically stable dataset and cannot have cause of variation.An out of control process will have so many variations and unstable data where the capability analysis cannot be performed.