Vol. 6, No. 6
here to view last month's Quality Quiz video.
Last month, Hyde N. Sikh, quality manager for Gowinta Plastics, found himself challenged by the concept of Cpk, when his own limited understanding was uncovered by his conscientious assistant, May Sendixen. Undaunted, he returns to the group of untrained inspectors whom he had selected to assure reduction in defects, offering them further training in capability analysis.
Always interested in audiovisual presentations that might provide a distraction from his own ineptitude, Sikh supplements his lecture with a demonstration of CHARTrunner charts that offer examples of the application of capability. One of his ragtag band of inspectors, Anita Jobb, has brought an Excel data set from the floor. Sikh decides that this data would be a good hands-on exercise for his trainees.
Jobb points out that the chart shows a negative Cpk. Sikh scoffs, saying that this is statistically impossible. “Just shows that manual calculations are always better than these computer programs,” he says, vowing to send the program back to the developer for refund.
Is Sikh right about the calculation that results in a negative Cpk?
Yes, he’s correct. It is indeed impossible to generate a negative
Cpk. The software must have a bug.
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