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

Karmond Geeya is a quality technician for Coastal Cruisers Company, an organization that produces outboard motors for marine vessels. Geeya is responsible for final assembly of these motors, including paint and trim, and for inspection of each product prior to shipping. The inspection includes a variety of characteristics that represent components of quality that C.C.C. is interested in producing. One of the inspection processes addresses paint applications, where defects are detected and recorded as data for analysis. If there is a blemish in the paint, for example, the motor is rejected by the inspector.

Geeya, who reads widely in his field, has discovered the usefulness of c-charts in statistical process control, and finds a useful symmetry in the possibilities of using this particular kind of chart for paint inspection. 'After all, ' he says wryly to his assistant, 'it seems only fitting that C.C.C. should be using c-charts. '

According to the textbook that Geeya has been consulting, c-charts should be used for discrete events. 'Well,' he muses, 'a pit in the paint job is certainly a discrete event. ' Other requirements include that the event should be defined by one limited area (one motor, for example), defects are independent of each other, and each type of defect occurs infrequently. These factors must be present because a c-chart follows what is known as a Poisson distribution, something that Geeya unfortunately read as 'poison. '

As he instructs his assistant about the use of c-charts, he reviews each of the assumptions, and the two of them decide that indeed, the analysis would be addressing discrete occurrences, the defects would be confined to a well-defined area (one motor), and that they are independent of each other. As it happens, some defect types are not particularly infrequent, but as Geeya points out, 'Who's to say what's infrequent and what's not? ' and they decide that frequency really does not have a direct impact on the analysis anyway.

Are Geeya's assumptions about the use of c-charts correct?

a) yes

b) no

 

Click here for a more complete video explanation

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