V4.2 - Control Charts for attributes

For variables

• Quality characteristics measured on a continuous numerical scale

For attributes

• Quality characteristics assuming only two, binary states:

  • defective/non-defective

  • conforming/non-conforming

  • go/no-go

  • good/bad

Many quality characteristics are not measured on a continuous or quantitative scale

• Each unit of product is judged as conforming or non-conforming (defective), on the basis of certain attributes.

Charts for attributes are less expensive

• because measurements are less complex to be performed.

Attributes charts are generally not as informative as variables charts (lower information content).

• Sample size is typically larger => at least n ≥ 20 to 25

Hard Gauges => Go / No-Go-Test

Defect => Auftreten einer spezifischen Nichtkonformität, die so schwerwiegend ist, dass sie die sichere oder wirksame Verwendung des Produkts erheblich beeinträchtigt.

Defective => Product unit with one or more defects.

For defectives

• (p) control chart for fraction nonconforming

• (np) control chart for number nonconforming

For defects

• (c) control charts for defects/ nonconformities

  • for every product unit, we count the No. of defects on it.

• (u) control charts for defects/nonconformities per unit.