Shewhart Control Charts for Attributes

c Control Chart (Number of Defective)

  • This control chart displays the total number of non-conformities/ defects in sub-groups (e.g., sites, time sequence such as daily production run, where each day is considered a sub-group).(
  • The c chart assumes that the sample size of all sub-groups is constant.
Example Graph

c Control Chart Plotted Using a Published Dataset*

c 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.

* Source of data: http://www.itl.nist.gov/div898/handbook/pmc/section3/pmc331.htm:

  • The observed number of defects in 25 inspection units: Inspecting 25 successive wafers, each containing 100 chips, where the wafer is the inspection unit.