Shewhart Control Charts for Attributes

u Control Chart

  • This control chart displays the normalized number of defects/non-conformities in samples that can have varying number of inspection units
  • Each data point is obtained by dividing the number of defects by the number of inspection units for each sample.
Example Graph

u Control Chart (Graphical Display of Defects per Inspection Unit)*

u Control Chart

Use of historical µ and σ values for calculating the control limits:

The plot options of this control chart provides the option of using sample or historical data for µ or/and σ in the computation of control limits

  • Whether a graph is plotted based on sample or historical data is differentiated by annotation on the upper X-axis of the graph.

* Example: 25 rolls of fabric, each contains 35m2 of material, and each unit is 1m2. Hence there are 35 units per roll, but the number of inspected areas/units varies from sample to sample:

  • Each data point is obtained by dividing the number of defects by the number of inspection units for each sample; e.g., in Roll 1, the areas of inspected units = 30m2, and the total number of defects =7, which translates to 0.23 defect per unit (see the position of the leftmost data point in the graph above).