Statistical Methods for Quality Improvement, Third Edition

CHAPTER 6

Control Charts for Attributes

It is not always possible or practical to use measurement data in quality improvement work. Instead, count data (often referred to as attribute data) are used where, for example, the number of nonconforming parts for a given time period may be charted instead of measurements charted for one or more quality characteristics. Although automatic measuring devices have greatly simplified the measurement process, it is still often easier to classify a unit of production as conforming or nonconforming than to obtain the measurement for each of many quality characteristics. Furthermore, attribute control charts are used in many applications, such as clerical operations, for which count data, not measurement data, occur naturally.

Before studying and implementing charts for the number of nonconforming units, it is important to remember that, ideally, nonconforming units should not be produced. Consequently, attempting to control the number of nonconforming units at a particular level would generally be counterproductive. The objective, of course, should be to continually reduce the number of such units. When used for that purpose, such charts can be of value in indicating the extent to which the objective is being achieved over time. Thus, such charts might be used in conjunction with measurement charts, rather than simply in place of them. If either type of chart could be used in a particular situation, it would be wasteful of information to use ...