Attribute Control Charts

Minitab Reference

An attribute control charts 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.

P chart

With a P chart, the center line is straight.

NP chart

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

 

Attributes Control Charts for Poisson Data


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.