What is an attributes control chart?

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.

Attributes control charts for binomial data

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 nonconforming units. The main difference between P and NP charts is the vertical scale.
  • P charts show the proportion of nonconforming units on the y-axis.
  • NP charts show the whole number of nonconforming units on the y-axis.
The chart that you choose does not affect which points are out of control.

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.

With a P chart, the center line is straight.

With an NP chart, the center line varies with subgroup size changes.

Attributes control charts for Poisson data

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 your nonconforming units. The main difference between U and C charts is the vertical scale.
  • U charts show the number of nonconformities per single unit on the y-axis.
  • C charts show the number of nonconformities per sample, which can include more than one unit on the y-axis.
The chart that you choose does not affect which points are out of control.
Note

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

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