An analysis of the long range operating characteristics of the MIL-STD-105D sampling scheme and some suggested modifications

Abstract

An integral part of the MIL-STD-105D scheme for sampling inspection by attributes is the transfer from normal inspection to reduced or tightened inspection when the historical record of inspected lots suggests unusually good or bad quality. The switching rules in MIL-STD-105D have been criticized, especially by Japanese manufacturers, as being too severe when what is defined as acceptable quality material is submitted. This paper examines the long range fraction of lots rejected for several MIL-STD-105D sampling plans by using the MIL-STD-105D switching rules, using a modification suggested by the Japanese Standards Association, and by using a second modification developed by the authors. The Japanese Standards Association switching rules are more complex than those in MIL-STD-105D. It is demonstrated that they lead to improved long-range properties for Normal-Tightened-Reduced schemes, but to poorer properties for Normal-Tightened schemes. A simplified set of switching rules is suggested, wherein the “limit numbers” in MIL-STD-105D are eliminated. In comparison to MIL-STD-105D, the simpler rules lead to a lower probability of rejection for good lots and a comparable probability of rejection for bad lots.