February 2002

Vol. 4, No. 2

Quality Quiz from Dr. Cleary

You're right!

Karmond Geeya, by disregarding one of the essential assumptions, is rendering the use of c-charts invalid. All of the four assumptions must be met. These assumptions are:

1. The occurrences of a defect type are discrete:

2. The area of opportunity (motor) is well defined;

3. The occurrence of one type of defect is not influenced by the occurrence of another type;

4. The probability of any one type of defect occurring is small.

Although assumption #4 is somewhat subjective, Geeya is incorrect in disregarding it. Without all four assumptions in place, he will produce an inaccurate (possibly poisonous) chart.

Below is an example of a c-chart that has met all of the conditions outlined in the four assumptions.


Copyright 2002 PQ Systems.

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