Attributes control charts in Minitab

Minitab offers several attribute control charts that plot defects or defectives. A defect refers to a quality characteristic and a defective unit refers to the overall product. A unit may have many defects, but the unit itself is either defective or nondefective. For example, a scratch on a metal panel is a defect. If several scratches exist, the entire panel may be considered defective.

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.

Attributes control charts with varying subgroup sizes

When the subgroup sizes are different:

• The control limits for both charts vary.
• The center line on the NP chart varies, but the center line on the P chart is straight.

The varying center line may make the chart more difficult to interpret. Traditional advice is to use a P chart if the subgroup sizes are different. However, you can use either chart.

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.