Westgard rule 13sstates 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 image, 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.
