Others Acceptance Sampling Techniques

Chapter 15 Introduction to Statistical Quality Control, 5th Edition by Douglas C. Montgomery.Copyright (c) 2005 John Wiley & Sons, Inc.

1


2


3


4

Advantages • Primary advantage of variables sampling plans is that the same operating-characteristic curve can be obtained with a smaller sample size than would be required by an attributes sampling plan• Second advantage is that measurement data usually provide more information than attributes data• Final point is that when acceptable quality level as are very small, sample sizes required by attributes sampling plans are very large

Disadvantages• Distribution of OC curve must be known

• Most standard plans assume distribution of quality characteristic is normal• A separate sampling plan must be employed for each quality characteristic that is being inspected• Possible to reject a lot even though the actual sample inspected does not contain any defective items


5


6


7


8

• Usual assumption is that the parameter of interest follows the normal distribution• If parameter of interest is not normally distributed, estimates of the fraction defective will not be the same as if normally distributed• Difference between estimated fraction defectives may be large when dealing with very small fractions defective


9


10


11


12


13


14


15

• MIL STD 414 is a lot-by-lot acceptance-sampling plan for variables• Focal point is the AQL which ranges from 0.04% to 15%• Five general inspection levels• Sample sizes are a function of the lot size and the inspection level• Provision is made for normal, tightened, and reduced inspection• Quality characteristic of interest is assumed to be normally distributed


16


17

Refer to the textbook for Table 15-2 (p. 697)


18


19


20

• It is not possible to move directly from an attributes sampling plan in the current MIL STD 105E to a corresponding variables plan in MIL STD 414 if the assurance of continued protection is desired for certain lot sizes and AQLs• Civilian counterpart of MIL STD 414 restores the match to attributes plans—that is, ANSI/ASQC Z1.9 is directly compatible with MIL STD 105E

• Makes it possible to start inspection by using an attributes scheme from MIL STD 105E, collect sufficient information to use variables inspection, then switch to variables scheme while maintaining the same AQL-code letter combination


21


22


23


24


25


26


27

• Two approaches may be used to form lots:1. Accumulate production at given points in the assembly process2. Mark off a given segment of production as a “lot”

• Continuous sampling plans address disadvantages of both approaches


28


29


30

Variations to the original Dodge CSP-1 have been designed to meet objections• Dodge and Torrey (1951) proposed plans to meet the objection that the occurrence of a single isolated defective unit sometimes does not warrant return to 100% inspection• Lieberman and Solomon (1955) have designed multi-level continuous-sampling plans to overcome the abrupt transition between sampling inspection and 100% inspection• Much of the work has been incorporated into MIL STD 1235C


31


32


33


34


35


36


37


38

Others Acceptance Sampling Techniques

Business

sampling inspection

attributes sampling

variables inspection

lot continuous sampling

lot acceptancesampling

separate sampling plan

current mil std

inspection lieberman