May  2002

Vol. 4, No. 5

Quality Quiz

Quality technician I. Michelle Shokked is exhausted from the daily grind of the quality lab, but can't afford to take a day off. However, she has persuaded her boss that she needs additional training in order to understand capability more thoroughly. Of course her intention is to sit in the back of the training class and catch up on her sleep, but she signs up for a one-day seminar at the local community college.

Professor Kaye Cabbillity leads a discussion about the importance of assuring that a process is in control, or stable, before undertaking capability study. Michelle has heard this before, and nods off during the discussion. When she is jolted awake by a flash on the overhead projector, she sees that the professor is demonstrating an example of negative Cpk, in which the process average (X-double bar) is greater than the upper specification limit. Michelle finds this interesting, since it suggests something with which she might impress her colleagues when she returns to the workplace.

Sure enough, when she returns to the plant, a co-worker comes to her with a case of negative Cpk, wondering how this can be. Smiling confidently, Michelle points out that a negative Cpk only means that the process average (X-double bar) must be greater than the upper spec limit. The co-worker doesn't understand why this is true, but Michelle is unable to clarify.

Should Michelle's technician accept her explanation as true, or is there a more accurate explanation?

A. Michelle Shokked knows her stuff. She is absolutely right this time.

B.The situation that the co-worker describes will arise only when the process average is less than the lower spec limit.

C.The only accurate response is to say that when there is a negative Cpk, the process average must be above the upper or below lower spec limits.

Copyright 2002 PQ Systems.

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