Deck 11: Process Capability

Question 1

How can someone studying a process know whether or not the process is under control? Why is this important?

Answer 1

A process is under control if the amount of variation present in the process remains constant over time. Process centering must also be constant over time. If both of these are constant, then process behavior is predictable enabling decisions to be made about process performance.

Question 2

Why can process capability indices only be calculated reliably when the process is stable?

Answer 2

When the process is stable, it is predictable. If the process is unpredictable (out-of-control) then we don't know what to expect in the future and the Cp and Cpk values are of little value.

Question 3

A customer has asked for process capability on a particular process. The control charts, created with a sample size of n = 5, in place show that the centerline (XDB) is 0.50349 with a range of 0.00014. The specifications for this process are 0.50350 +/- 0.00020 mm. What should the customer be told about process capability?

Answer 3

Question 4

For the following information, calculate Cp and Cpk.

Question 5

Dissatisfied with the results of their previous process improvement efforts, the Six Sigma team in question 8 has decided to make additional changes. Specifications for a bicycle hub measure 3.750 +/- 0.005 inches. Calculate Cp and Cpk for a XDB of 3.7503 and a σ of 0.0008.

Question 6

Specifications for a bicycle hub measure 3.750 +/- 0.005 inches. A Six Sigma project team has been improving the process that manufactures these hubs. They would like to compare three different machine settings using Cp and Cpk. Complete the following table by calculating and comparing these values.

Question 7

What is the relationship between 6 sigma, Cp and Cpk? What do each of these values tell us?

Question 8

Calculate and interpret the Cp and Cpk values for the following information.

Question 9

What does understanding the capability of the process do for Six Sigma practitioners?

Question 10

Stress tests are used to study the heart muscle after a person has had a heart attack. The team investigating stress tests turnaround time has managed to reduce the amount of time it takes for a doctor to receive the results of a stress test from 68 to 32 hours on average. The team had a goal of reducing test turnaround times to between 30 and 36 hours. Given that the new average is test turnaround time is 32 hours, with a standard deviation of s-bar = 1, and n = 9, calculate and interpret Cp and Cpk. Work this problem to whole numbers.