Most real processes in practice are no longer normally distributed in long-term analyses. ISO 22514-2 shows how to classify changes in the location and variation of a process. This standard also describes the resulting distribution and calculation methods for C_p and C_pk or P_p and P_pk. To simplify matters, the normal distribution serves as an approach to teach the basics of process capability.

1. A process capability index of C_p greater than or equal to 1.8 is demanded for a normally distributed filling volume. What is the maximum permissible standard deviation when the specification is (10.0 ± 0.02) cl?

• 0.0037 cl
• 0.0380 cl
• 0.0073 cl

2. Provided that your data are normally distributed, what is the error proportion if C_p = C_pk = 1?

• 0.27%
• 2.7%
• 99.73%

3. The evaluation of a process based on long-term process capability indices leads to the conclusion that your process is capable and stable according to VDA guidelines. It is adjusted to the tolerance centre. Which requirement does it meet?

• C_p greater than or equal to 1.33 and C_pk < C_p
• C_p greater than 1.33 and C_pk = C_p
• C_p = 1.33 and C_pk = 0

4. You calculate a pair of statistics in a process capability analysis for a two-sided quality characteristic. Which combination is (even theoretically) not possible?

• C_p = 1.66 and C_pk = 0
• C_p = 1.66 and C_pk = 1.66
• C_p = 1.66 and C_pk = 2

Our Q-QUIZ is published once a month. You can find the answer to all questions in the PDF file below.
This quiz was originally published in the August issue 2018 of the German magazine QZ Qualität und Zuverlässigkeit (http://www.qz-online.de/).