Shewhart attributes control charts

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.

• NP chart
  A np-chart is a type of control chart used to monitor the number of nonconforming units when measuring subgroups at regular intervals from a process.
• P chart
  A p-chart is a type of control chart used to monitor the proportion of nonconforming units when measuring subgroups at regular intervals from a process.
• C chart
  A c-chart is a type of control chart used to monitor the total number of nonconformities when measuring subgroups at regular intervals from a process.
• U chart
  A u-chart is a type of control chart used to monitor the average number of nonconformities per unit when measuring subgroups at regular intervals from a process.