Sample Size based on Square Root of (N) plus One


What about the sampling rule based on the "square root of (N) + 1"?

What are its origins?
Is this a valid sample size formula?




Arguments AGAINST



The article "The Square Root of N Plus One Sampling Rule.pdf" (attached below) by Pharmtech gives a good overview of the origins of this sampling rule, along with reviews of the assessment of its validity.

[Quality Focus article] "When is a sample not a sample.pdf" (attached below)

Article "WHO Pharmaceutical Technical Report Series 929.pdf" (see page 85 of 152) (attached below).




Arguments FOR



Pharmtech article "Statistical Solutions -- Square Root of (N) Plus One Sampling Plan.pdf" (attached below).



Other Considerations



Scanned article excerpt "Square Root N Plus One -- attributed to Dodge.pdf" (attached) attributes the sampling plan to Harold Dodge.


Other related information:



The Bottom Line ... ??


In general, I believe that there is enough question about its validity (or at least the circumstances under which it might be valid) that I would not advocate its use; particularly if other, more statistically valid and generally accepted methods are available.


 
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