Contents - Index

Varying Protection

Varying protection means that the protection provided by the sampling plan changes depending on circumstances.  The following types of sampling plans have varying protection:
  • ANSI Z1.4 - the switching rules in this standard use the results of previous inspections to determine which sampling plan to use for the current inspection.  Therefore the chances of passing are affected not only by the quality of the current lot, but also the quality of previous lots.
  • ANSI Z1.9 - the switching rules in this standard use the results of previous inspections to determine which sampling plan to use for the current inspection.  Therefore the chances of passing are affected not only by the quality of the current lot, but also the quality of previous lots.
  • Variables Single Sampling Plan For Defective Units - when standard deviation is unknown and 2 specs - the same defect rate can result from multiple combinations of the average and standard deviation which in turn have slightly different chances of passing.  Typically this effect is very small making the protection nearly constant.
     
    When the protection varies, a single OC Curve does not adequately describe the protection provided by the sampling plan.  Instead multiple OC curves are required including the Stationary OC Curve, Maximum OC Curve and Minimum OC Curve.  These OC curves are summarized using Max OC(LTPD) - Maximum Probability of Acceptance at LTPD, Min OC(AQL) - Minimum Probability of Acceptance at AQL, Rate of Convergence at LTPD, and Rate of Convergence at AQL.