I am aware that, in SPC, one can theoretically chose different control limits based on the number of standard deviations one wishes to use.

If one uses a control chart with control limits with fewer than three standard deviations - e.g. x-bar +- 2/1.128, do the corresponding Nelson rules - e.g. x number of observations below the mean, etc, change as well? The logical extension of reducing the control limits (and hence increasing the probability that rule 1 will be triggered), is that the sensitivity of the other rules should increase too. Is this the case?


1 Answer 1


Decreasing the amount of standard deviations increases the probability that a point is outside of the control limits. Using only 2 standard deviations will give 1/20 points as false positive. The non-conformances are then no longer useful, as there are too many false positives.

To detect process shift, detecting only points outside 3 sigma is typically enough. The other Nelson rules provide significantly diminishing returns and increase the false positive rate.
Source: https://www.qualitydigest.com/inside/statistics-column/when-should-we-use-extra-detection-rules-100917.html


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.