2. Introduction to Statistical Quality Control, 4th E
14-1. The Acceptance-Sampling
Problem
ā¢ Acceptance sampling is concerned with
inspection and decision making regarding
products.
ā¢ Three aspects of sampling are important:
1. Involves random sampling of an entire ālotā
2. Accept and Reject Lots (does not achieve
quality improvement) āLot sentencingā
3. Audit tool
3. Introduction to Statistical Quality Control, 4th E
14-1. The Acceptance-Sampling
Problem
ā¢ Three approaches to lot sentencing:
1. Accept with no inspection
2. 100% inspection
3. Acceptance sampling
4. Introduction to Statistical Quality Control, 4th E
14-1. The Acceptance-Sampling
Problem
Why Acceptance Sampling and Not 100%
Inspection?
ā¢ Testing is destructive
ā¢ Cost of 100% inspection is high
ā¢ 100% inspection is not feasible (require
too much time)
ā¢ If vendor has excellent quality history
5. Introduction to Statistical Quality Control, 4th E
14-1. The Acceptance-Sampling
Problem
14-1.1 Advantages and Disadvantages of Sampling
Advantages
ā¢ Less expensive
ā¢ Reduced damage
ā¢ Reduces the amount of inspection error
Disadvantages
ā¢ Risk of accepting ābadā lots, rejecting āgoodā lots.
ā¢ Less information generated
ā¢ Requires planning and documentation
6. Introduction to Statistical Quality Control, 4th E
14-1. The Acceptance-Sampling
Problem
14-1.2 Types of Sampling Plans
ā¢ There are variables sampling plans and attribute
sampling plans (this chapter concentrates on
attributes)
1. Single sampling plan
2. Double-sampling plan
3. Multiple-sampling plan
4. Sequential-sampling
7. Introduction to Statistical Quality Control, 4th E
14-1. The Acceptance-Sampling
Problem
14-1.3 Lot Formation
ā¢ Considerations before inspection:
ā Lots should be homogeneous
ā Larger lots more preferable than smaller lots
ā Lots should be conformable to the materials-
handling systems used in both the vendor and
consumer facilities.
8. Introduction to Statistical Quality Control, 4th E
14-1. The Acceptance-Sampling
Problem
14-1.4 Random Sampling
ā¢ The units selected for inspection should be
chosen at random.
ā¢ Random samples are not used, bias can be
introduced.
ā¢ If any judgment methods are used to select the
sample, the statistical basis of the acceptance-
sampling procedure is lost.
9. Introduction to Statistical Quality Control, 4th E
14-2. Single-Sampling Plans For
Attributes
14-2.1 Definition of a Single-Sampling Plan
ā¢ A single sampling plan is defined by sample size, n, and the
acceptance number c. Say there are N total items in a lot.
Choose n of the items at random. If at least c of the items are
unacceptable, reject the lot.
ā¢ N = lot size
ā¢ n = sample size
ā¢ c = acceptance number
ā¢ d = observed number of defectives
ā¢ The acceptance or rejection of the lot is based on the results
from a single sample - thus a single-sampling plan.
10. Introduction to Statistical Quality Control, 4th E
14-2. Single-Sampling Plans For
Attributes
14-2.2 The OC Curve
ā¢ The operating-characteristic (OC) curve measures
the performance of an acceptance-sampling plan.
ā¢ The OC curve plots the probability of accepting the
lot versus the lot fraction defective.
ā¢ The OC curve shows the probability that a lot
submitted with a certain fraction defective will be
either accepted or rejected.
11. Introduction to Statistical Quality Control, 4th E
14-2. Single-Sampling Plans For
Attributes
14-2.3 Designing a Single-Sampling Plan with a
Specified OC Curve
ā¢ Let the probability of acceptance be 1 - Ī± for lots
with fraction defective p1.
ā¢ Let the probability of acceptance be Ī² for lots with
fraction defective p2.
ā¢ Assume binomial sampling (with type-B OC curves)
is appropriate.
12. Introduction to Statistical Quality Control, 4th E
14-2. Single-Sampling Plans For
Attributes
14-2.3 Designing a Single-Sampling Plan with a
Specified OC Curve
ā¢ The sample size n and acceptance number c are the
solution to
ā ā
ā
=
ā ā
ā
=ā
=
ā
=
ā
c
0d
dn
2
d
2
c
0d
dn
1
d
1
)p1(p
)!dn(!d
!n
)p1(p
)!dn(!d
!n
1
Ī²
Ī±
13. Introduction to Statistical Quality Control, 4th E
14-2. Single-Sampling Plans For
Attributes
Example
ā¢ Consider constructing a sampling plan for which
ā p1 = 0.01
Ī± = 0.05
ā p2 = 0.06
Ī² = 0.10
ā N = 1000
ā¢ Using computer software or a graphical approach (using an
appropriate binomial nomograph) it can be shown that the
necessary values of n and c are 85 and 2, respectively.
14. Introduction to Statistical Quality Control, 4th E
14-4. Military Standard 105E
(ANSI/ASQC Z1.4 ISO 2859)
14-4.1 Description of the Standard
ā¢ Developed during World War II
ā¢ MIL STD 105E is the most widely used
acceptance-sampling system for attributes
ā¢ Gone through four revisions since 1950.
ā¢ MIL STD 105E is a collection of sampling
schemes making it an acceptance-sampling
system
15. Introduction to Statistical Quality Control, 4th E
14-4. Military Standard 105E
(ANSI/ASQC Z1.4 ISO 2859)
14-4.1 Description of the Standard
ā¢ Three types of sampling are provided for:
1. Single
2. Double
3. Multiple
ā¢ Provisions for each type of sampling plan
include
1. Normal inspection
2. Tightened inspection
3. Reduced inspection
16. Introduction to Statistical Quality Control, 4th E
14-4. Military Standard 105E
(ANSI/ASQC Z1.4 ISO 2859)
14-4.1 Description of the Standard
ā¢ The acceptable quality level (AQL) is a primary focal
point of the standard
ā¢ The AQL is generally specified in the contract or by the
authority responsible for sampling.
ā¢ Different AQLs may be designated for different types of
defects.
ā¢ Defects include critical defects, major defects, and minor
defects.
ā¢ Tables for the standard provide are used to determine the
appropriate sampling scheme.
17. Introduction to Statistical Quality Control, 4th E
14-4. Military Standard 105E
(ANSI/ASQC Z1.4 ISO 2859)
14-4.1 Description of the Standard
ā¢ Switching Rules
ā Normal to tightened
ā Tightened to normal
ā Normal to reduced
ā Reduced to normal
ā Discontinuance of inspection
18. Introduction to Statistical Quality Control, 4th E
14-4. Military Standard 105E
(ANSI/ASQC Z1.4 ISO 2859)
14-4.2 Procedure
1. Choose the AQL
2. Choose the inspection level
3. Determine the lot size
4. Find the appropriate sample size code letter from Table
14-4
5. Determine the appropriate type of sampling plan to use
(single, double, multiple)
6. Enter the appropriate table to find the type of plan to be
used.
7. Determine the corresponding normal and reduced
inspection plans to be used when required.
19. Introduction to Statistical Quality Control, 4th E
14-4. Military Standard 105E
(ANSI/ASQC Z1.4 ISO 2859)
Example
ā¢ Suppose a product is submitted in lots of size
N = 2000. The AQL is 0.65%. Say we wanted
to generate normal single-sampling plans.
ā¢ For lots of size 2000, (and general inspection
level II) Table 14-4 indicates the appropriate
sample size code letter is K.
ā¢ From Table 14-5 for single-sampling plans
under normal inspection, the normal inspection
plan is n = 125, c = 2.
20. Introduction to Statistical Quality Control, 4th E
14-4. Military Standard 105E
(ANSI/ASQC Z1.4 ISO 2859)
14-4.3 Discussion
ā¢ There are several points about the standard that
should be emphasized:
1. MIL STD 105E is AQL-oriented
2. The sample sizes selected for use in MIL STD 105E
are limited
3. The sample sizes are related to the lot sizes.
4. Switching rules from normal to tightened and from
tightened to normal are subject to some criticism.
5. A common abuse of the standard is failure to use the
switching rules at all.
21. Introduction to Statistical Quality Control, 4th E
14-4. Military Standard 105E
(ANSI/ASQC Z1.4 ISO 2859)
14-4.3 Discussion
ā¢ ANSI/ASQC Z1.4 or ISO 2859 is the civilian standard
counterpart of MIL STD 105E.
ā¢ Differences include:
1. Terminology ānonconformityā, ānonconformanceā, and
āpercent nonconformingā is used.
2. Switching rules were changed slightly to provide an option for
reduced inspection without the use of limit numbers
3. Several tables that show measures of scheme performance were
introduced
4. A section was added describing proper use of individual
sampling plans when extracted from the system.
5. A figure illustrating switching rules was added.