Vol. 11, No. 8
August 2009
PQ Systems
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Quality Quiz from Professor Cleary

Congratulations:
"C" is correct.

Click here for a more complete video explanation

Anna Gramm needs to review her P’s and Q’s.

Like p-charts, u-charts record attributes data. But the distinction lies in the difference between go/no go situations, where a product either has a defect or it does not, and nonconforming products, which may have several defects in a single product. It would be necessary to document the types of defects found, so the most frequently-recurring ones could be addressed. P-charts collect data relating to the proportion of rejected containers, while u-charts give information about how many individual defects are recorded in samples.

For example, manufactured parts that are the wrong dimension and tested with a go/no-go approach should be charted on a p-chart. Ice cream, on the other hand, might have too many chocolate chips, a sloppy label, and insufficient fill—all in the same container. Instead of recording the ice cream container as good or bad, more useful information would be gathered by identifying the kinds and number of defects in each batch of containers. This could be charted with a u-chart, and future analysis by means of Pareto charts.

Since types of defects are apparently not being recorded, the inspectors probably look for the most obvious kind of error, and stop looking for others once the carton has been rejected as defective.

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