Statistical Quality Control (SQC)

What is SQC

Statistical Quality Control (SQC) is a method of controlling the quality of production output using statistical methods. This increases the likelihood that the production facility operates at its full potential.

Statistical Quality Control is an integral component of OEE and is therefore a key driver of the effectiveness of YOUR facility. SQC can be applied to any process where the “conforming product” (product meeting specifications) output can be measured. Key tools used in SQC include control charts; a focus on continuous improvement; and the design of experiments.

Collecting SQC sample data

Using an integrated approach such as that provided by Ignition, we can capture statistical quality measurements automatically via the control systems, via electronic links to Laboratory Information Systems (LIMS), or via manual entry. Automatic prompting ensures accurate and timely collection of data for quality analysis.


Reporting and displaying SQC data

Quality data can be freely reported, and displayed in a multitude of formats including:

  • XBar – Range
  • XBar – S
  • Individual
  • Pareto
  • Histogram
  • P chart
  • NP chart
  • C chart
  • U chart

SQC Alerts

There are many well documented rules for detecting “out of control”, or non-random conditions, in manufacturing processes. For example, the Western Electric Rules were first published in the Statistical Quality Handbook in 1956, and the Nelson rules were published in 1984. These rules were first postulated by Walter A Shewhart in the 1920’s.

Some examples of SQC rules that are applied to data sample points are:

  • One point is more than 3 standard deviations from the mean
  • Nine (or more) points in a row are on the same side of the mean
  • Six (or more) points in a row are continually increasing (or decreasing)
  • Fourteen (or more) points in a row alternate in direction, increasing then decreasing
  • Two (or three) out of three points in a row are more than 2 standard deviations from the mean in the same direction
  • Four (or five) out of five points in a row are more than 1 standard deviation from the mean in the same direction
  • Fifteen points in a row are all within 1 standard deviation of the mean on either side of the mean
  • Eight points in a row exist, but none within 1 standard deviation of the mean, and the points are in both directions from the mean

OpMetriks solutions can provide these rules as standard features. In addition, any user defined rules may be applied to YOUR data samples to alert YOU when YOUR process is out of control.