Shewhart Control Charts for Defect Data

Many quality assessment criteria for manufactured goods are not of the variable measurement type. Rather, some quality characteristics are more logically defined in a presence-of or absence-of sense. In P/M, examples include chipped gear teeth, discolored plating, scratched surfaces, and powder accumulation on parts.

Such nonconformities or defects are often observed visually or according to some sensory criteria and cause a part to be defined simply as a defective part. In these cases, quality assessment is referred to as being made by attributes.

Many quality characteristics that could be made by measurements (variables) are often not done as such in the interest of economy. A go/no-go gage can be used to determine whether or not a variable characteristic falls within the part specification. Parts that fail such a test are simply labeled defective. Attribute measurements can be used to identify the presence of problems, which can then be attacked by the use of A and R control charts. The following definitions are required in working with attribute data:

• Defect: A fault that causes an article or an item to fail to meet specification requirements. Each instance of the lack of conformity of an article to specification is a defect or nonconformity.

• Defective: An item or article with one or more defects is a defective item.

• Number of defects: In a sample of n items, c is the number of defects in the sample. An item may be subject to many different types of defects, each of which may occur several times.

• Number of defectives: In a sample of n items, d is the number of defective items in the sample.

• Fractional defective: The fractional defective, p, of a sample is the ratio of the number of defectives in a sample of the total number of items in the sample. Therefore, p = d/n.

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