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.
You can use the P chart to plot your nonconforming units. P charts show the proportion of nonconforming units on the y-axis.
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.
You can use the U chart to plot your nonconforming units. U charts show the number of nonconformities per single unit on the y-axis.