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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.

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Quality Control

Acceptance Sampling Systems


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Lot-by-Lot Acceptance Sampling Plans for Attributes

Devised in 1942 at Bell Labs

Later became MIL-STD-105E

ISO-2859

ANSI/ASQ Z1.4

Wording & terminology changes

Additional tables added


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ANSI/ASQ Z1.4

This standard is applicable to:

End items

Components and raw materials

Operations

Materials in process

Supplies in storage

Maintenance operations

Data or records


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ANSI/ASQ Z1.4

Intended to be used for a continuing

series of lots, but may be designed for

isolated lots.

Standard provides for single, double, and

multiple sampling plans.

Provision is also provided for normal,

tightened, or reduced inspection.

Plan is specified by the AQL, and sample-

size code.


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ANSI/ASQ Z1.4

Normal inspection is used at the start of

inspection with changes being a function of

recent quality history.

Tightened inspection:

Generally used when producer’s recent

quality history has deteriorated.


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ANSI/ASQ Z1.4

Reduced inspection:

Used when the producer’s recent quality

history has been exceptionally good.

Decision concerning what type of plan to use is

left to the responsible authority.

Nonconformities are classified such as critical,

major…


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ANSI/ASQ Z1.4


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Sample Size Code Letters


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Comparison of Inspection levels

III

1086

Percent Nonconforming (100Pa)

100

95

80

60

40

20

0

100=10

= 0.05

Per

cent

of L

ots

Acc

epte

d (1

00P

a)

42

II I

I = 1/2 n

II = n

III = 2n


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ANSI/ASQ Z1.4

AQL

The AQL is the most important part of the

standard since the AQL and sample size dictate

the plan used.

AQL is defined as the maximum percent

nonconforming that can be considered

satisfactory as a process average.

Satisfactory = Producer’s risk, α

Usually 0.05


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ANSI/ASQ Z1.4

AQL

AQL is specified by contract or

responsible authority.

AQL may be determine from

historical data, empirical judgment,

engineering information,

experimentation, producer’s

capability, consumer’s

requirements.


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ANSI/ASQ Z1.4

AQL

AQL is specified by contract or responsible

authority.

AQL may be determine from historical data,

empirical judgment, engineering information,

experimentation, producer’s capability,

consumer’s requirements.


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ANSI/ASQ Z1.4

Sample Size

Determined by lot size and inspection

level

Use of sample-size code

Inspection level is determined by the

responsible authority.


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ANSI/ASQ Z1.4

Sample Size

Different levels of inspection provide

approximately the same protection to the

producer, but different protections to the

consumer.

Plan provides for “special” levels to be

used where relatively small sample sizes

are necessary and large sampling risks

must be tolerated. (S-1, S-2, S-3, S-4)


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Implementation

Determine lot size

Determine inspection level

Find sample-size code letter in table

Determine AQL

ANSI/ASQ Z1.4


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Implementation cont’d.

Determine type of sampling plan

Find sampling plan in appropriate table

Start with normal inspection and change

to tightened or reduced based on

switching rules

ANSI/ASQ Z1.4


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What an AQL means is that as long as a

supplier maintains his/her process average (%

defective) at the assigned AQL or lower, there

is a very high probability that shipments from

that supplier, when inspected using ASQ Z1.4

sampling plans, will be accepted.

AQL


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By the same token, there is a very high

probability that shipments, when inspected

using ASQ Z1.4 sampling plans, will be rejected

if a supplier's process average (% defective)

remains higher than the assigned AQL.

AQL


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Single sampling plans

Start by identification of AQL, lot size,

inspection level, and type of sampling

plan.

Locate appropriate plan parameters in

tables.

Double and Multiple Sampling

Similar procedures as single sampling

ANSI/ASQ Z1.4


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Normal, Tightened, and Reduced Inspection

Start with normal inspection

Switching Procedures

Normal to tightened

Institute when 2 out of 5 consecutive lots not accepted

on original inspection

Tightened to normal

Institute when 5 consecutive lots accepted

If not, then discontinue inspection under this plan

ANSI/ASQ Z1.4


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Normal to reduced

Institute when: (all of the following)

Preceding 10 lots on normal inspection have

been accepted

Total nonconforming in preceding 10 samples

is less than number identified in Table 10-5.

Production is at a steady rate

Reduced inspection is considered desirable by

responsible authority

ANSI/ASQ Z1.4


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Reduced to normal

Institute when: (any of the following)

A lot is not accepted

Sampling procedure terminates with neither

acceptance or rejection criteria have been met

Production is irregular or delayed

Other conditions

ANSI/ASQ Z1.4


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Standard is designed for use where units of product

are produced in a continuing series of lots or

batches.

If a sampling plan is desirable for a lot or batch of an

isolated nature, it should be chosen based on the

Limiting Quality (LQ) and consumer’s risk, b.

These tables are included in the standard, but not

the textbook.

Or use ANSI/ASQ Standard Q3

ANSI/ASQ Z1.4


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Two Schemes:

1. Used for lots that are isolated or mixed or

that have an unknown history as far as all

parties know.

Lot size and LQ must be known.

Nominal values of the LQ are based on

b=0.10



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Two Schemes:

2. Used when a vendor is producing a

continuous stream of lots and sends one

or a few to a customer who will consider

them as isolated lots.

Occurs frequently when purchasing

small quantities of raw materials.


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