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Westgard Rule 13S

  • Westgard rule 13s states that if a control is greater than ± 3 standard deviations from the mean, it should be rejected and rerun.
  • This is because either a random error or a very large systematic error has occurred, as less than 1% of all test values exceed ± 3SD.
  • In the accompanying example, the control for Day 13(noted by the arrow) is greater than +3SD from the mean. Consequently, the 13s rule applies and the run is rejected. Troubleshooting must be performed before further testing can be done.