Vol. 11, No. 5
May 2009
PQ Systems
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Quality Quiz from Professor Cleary

"B" is correct.

Click here for a more complete video explanation

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 (one patient’s sample) 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 even though, as you can see, it is out of control.


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