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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
Quality Control
Acceptance Sampling Systems
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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…
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
ANSI/ASQ Z1.4
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
Sample Size Code Letters
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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)
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
Implementation
Determine lot size
Determine inspection level
Find sample-size code letter in table
Determine AQL
ANSI/ASQ Z1.4
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
Switching Procedures
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
Switching Procedures
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
Standard is used for inspection of isolated
lots by attributes.
Provides indexed tables by Limiting
Quality, LQ.
ANSI/ASQ Standard Q3
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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
ANSI/ASQ Standard Q3
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.
All rights reserved
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.
ANSI/ASQ Standard Q3
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