Attributes control charts plot quality characteristics that are not numerical (for example, the number of defective units, or the number of scratches on a painted panel).
It is sometimes necessary to simply classify each unit as either conforming or not conforming when a numerical measurement of a quality characteristic is not possible. In other cases, it is convenient to count the number of nonconformities rather than the number of nonconforming units. A unit may have a number of nonconformities without classing the unit as nonconforming. For example, a scratch on a painted panel may be a nonconformity but only if several such scratches exist would the
entire panel be classed as a nonconforming unit.