Shewhart Control Charts for Variables

X(bar) S Control Chart

  • Control limits for X(bar) S control chart are calculated as shown below.

Calculating Control Limits for X(bar) S Control Chart

Example Graph

X(bar) S Control Chart Concerning Semiconductor Processing*

Shewhart X(bar) S 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.

* Engineering Statistics Handbook. Process and Product Monitoring and Control. http://www.itl.nist.gov:

  • A case study of a lithography process that included measurements from five sites on each wafer, three wafers in a cassette (of equal sample size) and thirty cassettes of wafers. The quality characteristic of interest is the width of a line ("Raw Line Width").