A new variables sampling plan for normally distributed lots with unknown standard deviation and double specification limits

We deal with single sampling by variables with two-way-protection in case of normally distributed characteristics with unknown variance. Givenp ₁(AQL),p ₂ (LQ) and α, β (risks of errors of the first and the second kind), there are two well-known methods of determining the corresponding sampling plans. Both methods are based on an approximation of the OC. Therefore these plans are only approximations, the true risks α and β are not known exactly. In section II we present a new sampling scheme based on an estimatorp for the percent defectivep. We give an exact formula for the OC. Thus we are able to determine these plans exactly without any approximations.