Shewhart Control Charts for Attributes

np Control Chart (Number of Defective)

  • This control chart displays the number of defective/non-conforming items in samples of equal size.(
  • For an np chart, the process data must be stored as actual number of defective/non-conforming; in addition, the sample size must be constant.
Example Graph

np Control Chart Plotted Using a Published Dataset*

np 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/pmc332.htm:

  • The location of chips on a wafer is measured on 30 wafers. On each wafer 50 chips are measured and a defective is defined whenever a mis-registration, in terms of horizontal and/or vertical distances from the center, is recorded.