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61193-3 © IEC:2013
Because
of
the zero accept number, the idea of combining lots under the c = 0 plans may arise
because of the zero accept number. Aside from experience, which has shown that,
in
fact,
considerable savings can be derived, one should consider the following:
• if the quality is very bad, acceptance numbers greater than zero will not be of much help;
• to allow acceptance numbers greater than zero
in
the plan, one is in effect authorizing an
inspector to accept parts which may not
be
usable;
• the zero acceptance number forces a review
of
any defectives by quality assurance
personnel
in
order to enable proper withdrawal of the defectives;
• if zero defects are to be achieved, it should be obvious that defectives should not knowingly
allowed to be shipped.
The c = O plans were essentially designed to be equal or greater
in
consumer and average
outgoing quality limit protection. Within a particular column of the details shown in Table 2
representing the index value, the operating characteristic curves actually differ for the most
part between c = O and c > O plans, especially as the lot size increases. The reason for this
common feature, in addition to satisfying the statistical relationship, is that it is generally
consider
ed
more practical to obtain greater protection on larger lot sizes. Table 3 provides
guidance to selection of sample sizes and comes from the standards developed for printed
circuit board and laminate characteristic requirements. Table
A.1
of Annex A provides a
consensus sampling plan from IEC 62326-4 that identifies the different product characteristics,
the number of samples that should be taken for performance levels
A,
B,
and C, and the risk
management index va
lu
e to be used from Table 2.
Table 3 - Sample size selection guideline
Defects
Attributes
Critical
Major Minor
Critical 0, 1
1,0
2,5
Major 1,0 2,5
4,0
Minor 2,5
4,0
6,5
The use of constant sample sizes often results in a combination of over-inspection and under-
inspection. For a broad range of lot sizes
in
general, however,
in
order to develop
an
inspection
strategy,
an
evaluation should
be
made as to the attribute classification (critical, major, minor).
This listing of comparisons should identify the risk management index value shown
in
the
Table 2 and should allow the c = 0 plans to be used when
a) manufactured parts are expected to completely conform to specification requirements,
b) less inspection is desired
on
less critical characteristics,
c) sampling
is
performed because 100 % inspection on all attributes of all units of products is
impractical,
d) inspections are not a
ll
owed to knowingly accept non-conforming products,
e) auditing
is
required for assurance of process validation, potential transit damage,
certification of suppliers, or inventory verification.
5 Classification of attributes
5.1
General
Attributes are classified as part of the process
for
individual and/or grouped attributes
for
inspection.
selection of sampling plans applied to
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