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.

You can use either the P chart or the NP chart to plot your defective
units. The main difference between P and NP charts is the vertical scale.

- P charts show the proportion of defective units on the y-axis.
- NP charts show the whole number of defective units on the y-axis.

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.

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 either the U chart or the C chart to plot the number of
defects. The main difference between U and C charts is the vertical scale.

- U charts show the number of defects per unit on the y-axis.
- C charts show the number of defects per sample, where each item can have multiple defects.

Minitab does not create C charts for data with different numbers of units per subgroup.