Minitab offers several attribute control charts that plot nonconformities (defects) or nonconforming units (defectives). A nonconformity refers to a quality characteristic and a nonconforming unit refers to the overall product. A unit may have many nonconformities, but the unit itself is either conforming or nonconforming. For example, a scratch on a metal panel is a nonconformity. If several scratches exist, the entire panel may be considered nonconforming.
Select your attribute control chart based on whether your data follow a binomial or a Poisson distribution.
Values for binomial data are classified into one of two categories such as pass/fail or go/no-go. Binomial data are often used to calculate a proportion or a percentage, such as the percentage of sampled parts that are defective.
Values for Poisson data are often counts of defects or events. Poisson data are often used to model an occurrence rate, such as defects per unit.
Minitab does not create C charts for data with different numbers of units per subgroup.