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What is Acceptable Quality
Level (AQL)?
Acceptable Quality Level (AQL): This is the
numerical definition of a good lot, and can
be associated with the producer's risk.
ANSI/ASQC Standard A2 (1987) describes AQL
as "the maximum percentage or proportion of
nonconforming items or number of
nonconformities in a lot or batch that can
be considered satisfactory as a process
average." The AQL value refers to the
process average that, if kept stable at this
value, would result in accepting the
majority of lots.
AQL can be applied as a quality standard
with tighter or looser inspection criteria
based on the expected product quality.
When a process meets the AQL value or
better, the manufacturer will have the most
protection against unnecessary or improper
rejection of product lots.
When there is an elevated suspicion about
the quality of a product lot, or when lots
have a history of falling below the AQL
value, higher standards of inspection are
adopted to add additional protection.
Conversely, when there has been a history of
high or at least acceptable product quality,
inspection practices are more relaxed.
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The procedures used to formulate a sampling
plan using the ATQC-STD-101A are as follows:
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Decide on the
type of Sampling
(e.g., single) |
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Decide on the
state of
Inspection
(e.g., normal) |
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Decide on the
type of
Inspection Level
(usually Level
II) |
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Decide on Lot
Size (e.g., lot
of 1000
products) |
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How to use the following Sample Size
table & Sampling Plan table:
There are seven different inspection levels:
three general inspection levels, and four
special inspection levels. Each level is
situation-specific and will require a
different sampling size than the other
levels.
Three general inspection levels are provided
in the standard: Levels I, II, and III. They
provide approximately the same degree of
protection to the producer by rejecting on
average about 100% of the lots that have a
level of non-conformance given by AQL.
Within the chart, as the inspection level
goes up (from I to III), it provides a more
secure level of measurement to detect
defects and determine the quality of your
products.
Special levels (S1, S2, S3 and S4) usually
apply to situations with a relatively small
sampling size, or where high risk is
involved. Inspections that would use these
levels include destructive, time consuming
inspections or inspections for the
repetitive process industry (screw,
stamping, welding, etc...).
As for AQL index, inspections become tighter
as the digits in the sampling size become
smaller.
From the TABLE FOR SAMPLE SIZES,
ATQC-STD-101A for Lot Size (3201-10000)
and Inspection Level II; the corresponding
sample size code letter is “L.”.
Looking at the SINGLE SAMPLING PLANS FOR
NORMAL INSPECTION (TABLE OF ATQC-STD-102A),
the code letter L, combined with an AQL of
2.5%, corresponds to a required sampling
size “n” of 200 and a “c” value of 10. This
means that if no more than10 defectives are
found in a sample of 200 items, the lot will
be accepted.
TABLE FOR SAMPLES SIZES,
ATQC-STD-101A
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Click
here or on image to
enlarge. |
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SINGLE SAMPLING PLANS FOR NORMAL
INSPECTION (TABLE OF ATQC-STD-102A)
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Click
here or on image to
enlarge. |
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