Attributes Control Charts Overview
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Attributes control charts are similar in structure to variables control charts, except that they plot statistics from count data rather than measurement data. For instance, products may be compared against a standard and classified as either being defective or not. Products may also be classified by their number of defects.

As with variables control charts, a process statistic, such as the number of defects, is plotted versus a sample number or time. Minitab draws a center line at the average of the statistic being plotted for the time being charted. Minitab also draws two other lines - the upper and lower control limits -  3s above and below the center line, by default.

Control charts for defectives

You can compare a product to a standard and classify it as being defective or not. For example, a length of wire either meets the strength requirements or not. The control charts for defectives are:

·    P Chart, which charts the proportion of defectives in each subgroup

·    Laney P' Chart, which charts the proportion of defectives in each subgroup, while correcting for overdispersion or underdispersion.

·    NP Chart, which charts the number of defectives in each subgroup

See [1], [3], [7], [8], [10], [11], and [12] for more discussion of these charts.

Control charts for defects

When a product is complex, a defect does not always result in a defective product. Classifying a product by its number of defects is sometimes more convenient. For example, you might count the number of scratches on the surface of an appliance. The control charts for defects are:

·    U Chart, which charts the number of defects per unit sampled in each subgroup. Use U Chart when the subgroup size varies.

·    Laney U' Chart, which charts the number of defects per unit sampled in each subgroup, while correcting for overdispersion or underdispersion.

·    C Chart, which charts the number of defects in each subgroup. Use C Chart when the subgroup size is constant.

For example, if you were counting the number of flaws on the inner surface of a television screen, C Chart would chart the actual number of flaws, while U Chart or Laney U' Chart would chart the number of flaws per square inch sampled.

See [1], [3], [7], [8], [10], [11], and [12] for more discussion of these charts.