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Related Experiment Videos

Probability and the Westgard Rules.

Tomás A Carroll1, Heather A Pinnick, Wallace E Carroll

  • 1Phillips Petroleum Corporation, Borger, Texas, USA.

Annals of Clinical and Laboratory Science
|March 29, 2003
PubMed
Summary
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Shewhart's three standard deviation (3S) limits, adapted for clinical labs, signal when tests are out of control. Exceeding these limits indicates a need for corrective laboratory action to ensure accurate results.

Area of Science:

  • Clinical Laboratory Science
  • Quality Control
  • Statistical Process Control

Background:

  • Shewhart introduced three standard deviation (3S) limits for industrial quality control.
  • Levey, Jennings, Henry, and Westgard adapted these limits for clinical laboratory medicine.
  • Westgard developed a rule-based system for assessing laboratory test control status.

Purpose of the Study:

  • To mathematically explain Shewhart's "beyond 3S" concept.
  • To demonstrate how exceeding 3S limits indicates a need for corrective action in clinical laboratories.

Main Methods:

  • Mathematical explanation of Shewhart's three standard deviation (3S) limits.
  • Analysis of the probability of exceeding 3S limits (> 1 in 370 chance).

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Main Results:

  • Exceeding three standard deviation (3S) limits has a probability greater than 1 in 370.
  • This statistical threshold serves as a mathematical basis for identifying "out of control" laboratory tests.

Conclusions:

  • Shewhart's "beyond 3S" criterion is a statistically valid indicator for corrective action in clinical laboratories.
  • The mathematical framework supports the use of 3S limits for ensuring the quality and reportability of laboratory test results.