|An AQL Primer for Purchasing
We advise purchasing agents to exercise caution if they specify an AQL based standard
acceptance sampling plan for use with a purchase. Acceptance sampling has as its purpose
to prevent delivery of an off-quality lot -- should it unfortunately be manufactured in
the first place. When the purchaser specifies the AQL for an AQL based standard acceptance
sampling plan with the belief that AQL protects them, he or she is mistaken. This is
The term AQL has two meanings.
(1) STATISTICAL MEANING
OF AQL: -- Statistically, AQL is a quality level on the x-axis of an
operating characteristic curve (oc curve).
AQL is the x-coordinate of the producer's point. For the example above,
AQL=0.018. Also, for this example, the probability of acceptance at the producer's point
Note that the oc curve also contains a consumer's point which designates
the quality that would NOT pass the sampling plan. The quality level at the consumer's
point is RQL (Rejectable Quality Level). For the example above, RQL=0.093. Also, for this
example, the probability of acceptance at the consumer's point is Pa=0.05.
AND Z1.4 MEANING OF AQL: -- These standards instruct that AQL is the
quality level to be specified by the consumer... (See sec. 4.3 of Z1.4.)
THIS IS A CONTRADICTION: -- The statistical
definition of AQL associates it with the producer's point and the need of the producer to
accept lots that have been manufactured to the AQL level, while the Military and
Z-Standard instructions call for the consumer to specify AQL. The contradictory meanings
of AQL arise because the Military and Z-standards are not organized according to the
SOURCE OF CONFUSION: --
No wonder so many people are confused - the Military and Z-standards reverse the meaning
of producer and consumer. No wonder many purchasing agents wrongly believe that by
specifying AQL they are enhancing their (consumer's) need to not accept off-quality lots.
The practical consequence of all this is that if you study Mil-Std-105E or ASQC/ANSI-Z1.4
to learn about AQL, you will learn a system, but you will also be learning how to specify
sampling plans that are not designed to protect the consumer's interest.
SAMPLING: -- In order for an acceptance sampling plan to be
statistically valid, the person that specifies the plan must know and accept the
probabilities and risks of that plan. You can learn about this methodology to
protect both the consumer and the producer by reading and printing the 20-page tutorial on
modern acceptance sampling.
A shorter explanation of the correct way to develop an acceptance sampling plan explains
the operating characteristic (oc) curve.
ACCEPTANCE SAMPLING: -- Modern acceptance sampling practice is
supported by acceptance sampling software that uses
both the consumer's point and the producer's point of the oc-curve.
FORUM -- You can collaborate with others to get additional questions
discussed at the sampling plan forum .
Author -- Stan Hilliard, CQE,CRE,CQA,PE