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6.
Process or Product Monitoring and Control
6.3. Univariate and Multivariate Control Charts
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| Attributes
data arise when classifying or counting observations
Types of attribute control charts |
The Shewhart control chart
plots quality characteristics that can be measured and expressed numerically.
We measure weight, height, position, thickness, etc. If we cannot
represent a particular quality characteristic numerically, we then resort
to classifying an item that is inspected as either conforming or nonconforming
to the specifications of that characteristic.
The terminology "conforming" and "nonconforming" is synonymous with the terminology "non defective" and "defective". Quality characteristics of that type are called attributes. Examples of quality characteristics that are attributes are the number of failures in a production run, the proportion of malfunctioning wafers in a lot, the number of people eating in the cafeteria on a given day, etc. Control charts dealing with the number of defects or nonconformities are called c charts (for count) Control charts dealing with the proportion or fraction of defective product are called p charts (for proportion) . There is another chart which handles defectives per unit, called the u chart (for unit). This applies when we wish to work with the average number of nonconformities per unit of product. For additional references, see Woodall (1997) which reviews papers showing examples of attribute control charting, including examples from semiconductor manufacturing such as those examining the spatial depencence of defects |
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