The standards work of

ISO/TC 69/SC 5

‘Acceptance sampling’

David Baillie

Chairman, ISO/TC 69/SC 5

ISO/TC 69/SC 5 has published 3 Technical Reports and 18 standards. 4 of these standards are presently under revision and 2 new standards are presently under development. 4 further standards are planned for the future.

One of the purposes of this presentation is to gather ideas, if any, for potential new acceptance sampling standards.

ISO 2859 Sampling procedures

for inspection by attributes

The ISO 2959 series presently consists of

6 parts, namely Parts 1 to 5 and Part 10.

ISO 2859 series (continued)

ISO 2859-10

Introduction to the ISO 2859 series of

attribute sampling standards

ISO 2859 series (continued)

ISO 2859-1:1999 Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection

Contains single sampling, double sampling and multiple sampling plans, both for percent nonconforming items and for nonconformities per 100 items

Applicable to a continuing series of lots.

ISO 2859 series (continued)

ISO 2859-2:1985

Sampling plans indexed by limiting quality

(LQ) for isolated lot inspection

This standard is presently under revision

at DIS ballot stage.

ISO 2859 series (continued)

ISO 2859-3:2005

Skip-lot sampling procedures

ISO 2859 series (continued)

ISO 2859-4:2002

Procedures for assessment of declared

quality levels

ISO 2859 series (continued)

ISO 2859-5:2005

System of sequential sampling plans

indexed by acceptance quality limit (AQL)

for lot-by-lot inspection

ISO 3951 series: Sampling

procedures for inspection by

variables

The ISO 3951 series presently consists of

5 parts, namely Parts 1 to 5.

ISO 3951 series (continued)

ISO 3951-1:2005 Specification for single

sampling plans indexed by acceptance

quality limit (AQL) for lot-by-lot inspection

for a single quality characteristic and a

single AQL

ISO 3951 series (continued)

ISO 3951-2:2006 General specification

for single sampling plans indexed by

acceptance quality limit (AQL) for lot-by-lot

inspection of independent quality

characteristics

ISO 3951 series (continued)

ISO 3951-3:2007 Double sampling

schemes indexed by acceptance quality

limit (AQL) for lot-by-lot inspection

ISO 3951 series (continued)

ISO 3951-4 (under DIS ballot)

Procedures for assessment of declared

quality levels

ISO 3951 series (continued)

ISO 3951-5:2006 Sequential sampling

plans indexed by acceptance quality limit

(AQL) for inspection by variables (known

standard deviation)

Sequential sampling plans indexed

by producer’s and consumer’s risk

points

In order to satisfy a need for sequential

sampling plans having fixed maximum

risks at two quality levels, two standards

have been developed, one for attributes

and one for variables.

The plans can be used for acceptance

sampling or hypothesis testing.

Sequential sampling plans indexed

by producer’s and consumer’s risk

points (continued)

ISO 8422:2006 Sequential sampling plans

for inspection by attributes

Sequential sampling plans indexed

by producer’s and consumer’s risk

points (continued)

ISO 8423:2008 Sequential sampling plans

for inspection by variables for percent

nonconforming (known standard deviation)

Accept zero standards

Because of the preference of many

industries to rely on sampling plans with

acceptance number zero (despite the poor

ability of such plans to discriminate

between good and bad quality levels when

used in stand-alone mode), ISO/TC 69/SC

5 has developed and published three

standards that depend strongly on such

plans.

Accept zero standards (continued)

ISO 18414:2006 Acceptance sampling procedures by attributes ― Accept-zero sampling system based on credit principle for controlling outgoing quality

Uses accept-zero plans on a continuing series in a structured way so that the average outgoing quality limit can be guaranteed. Method has the advantage that large lots need never be 100% inspected.

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Accept zero standards (continued)

ISO 21247:2005 Combined accept-zero sampling systems and process control procedures for product.

Based on MIL-STD-1916, provides a set of accept-zero sampling systems and procedures for planning and conducting inspections to assess quality and conformance to specified requirements.

Accept zero standards (continued)

ISO 28801:2002 (presently under 2nd DIS ballot)

Double sampling plans by attributes with minimal

sample sizes, indexed by producer’s risk quality

(PRQ) and consumer’s risk quality (CRQ)

Accept and reject numbers 0 and 2 at the first

sample, but 1 and 2 for the combined first and

second samples.

Parts per million standardISO 14560:2004 Acceptance sampling procedures by attributes ― Specified quality levels in nonconforming items per million

Specifies, for quality levels expressed as nonconforming items per million items, procedures for estimating the quality level of a single entity (e.g. a lot) and, when the production process is in statistical control, for estimating the process quality level based on evidence from several samples.

ISO 14560:2004 (continued)

Procedures are also specified for using

this information when selecting a suitable

sampling plan so as to verify that the

quality level of a given lot does not exceed

a stated limiting quality level (LQL). For

the case where no prior sample data is

available, guidance is given for presuming

a process quality level in selecting a plan.

Allocation of priorities principle

standards

ISO 13448 Acceptance sampling procedures based on the allocation of priorities principle (APP)

Part 1 (2005): Guidelines for the APP approach

Part 2 (2004): Coordinated single sampling plans for acceptance sampling by attributes

Uses knowledge of the history of the process and the level of accreditation in a semi-Bayesian way to reduce the required sample sizes.

Random sampling

ISO 24153:2009 Random sampling and randomization procedures

Defines procedures for random sampling and randomization. Several methods are provided, including approaches based on mechanical devices, tables of random numbers, and portable computer algorithms.

Technical reports (TRs)

ISO/TR 8550: Guidance on the selection and

usage of acceptance sampling systems for

inspection of discrete items in lots

consists of three Parts, all published in 2007:

Part 1: Guide to acceptance sampling

Part 2: Sampling by attributes

Part 3: Sampling by variables

Current work

• Revision of ISO 3951-1 and ISO 3951-2 to

provide a better match between the OC

curves for single sampling by variables

and the OC curves for the corresponding

single sampling attributes plans of ISO

2859-1.

(New work item proposals to be made in

September and October 2010)

Current work (continued)

• ISO 28801 revision

This revision is primarily to introduce plans

with a producer’s risk of 5% and a

consumer’s risk of 10%.

(The current edition only has plans for the

cases where both risks are equal to either

5% or 10%.)

Current work (continued)

• ISO 3951-4 Procedures for assessment

of declared quality levels

This standard will be a variables

counterpart to the attributes standard ISO

2859-4.

It is presently under DIS ballot.

Future work

Sampling by attriables

This standard will accommodate the situation in

acceptance sampling where a class of quality

characteristics includes at least one attribute and

at least one variable.

(This is one reason why ISO 3951-1 and ISO

3951-2 are being revised, to harmonise the

attributes and variables OC curves for use in

attriables sampling.)

Future work (continued)

Audit sampling

Now that the accountancy profession

acknowledges that it is better if sampling

inspection were not discretionary, it is a

good opportunity for ISO to fill the need for

an appropriate international standard.

Future work (continued)

• ISO 3951-10 Introduction to the ISO 3951

series of attribute sampling standards

A guidance document will help the user to

navigate the ISO 3951 series (mirroring

ISO 2859-10 for sampling by attributes).

Future work (continued)

Continuous sampling plans

The problem of existing continuous sampling plans is that the varying inspection load is uneconomic. Methodology has been developed by Wadsworth and Wasserman for basing the sampling plans on cumulative sums, as a result of which the inspection load remains constant.

Wadsworth, H.M., and Wasserman, G., 1987. A Modified CUSUM Procedure as A Continuous Sampling Scheme. In Frontiers in Statistical Quality Control 3, pp. 179-196. GmbH, Co., Heidelberg, Germany, Physica-Verlag

Future work (continued)

Sequential sampling plans by variables for

unknown process standard deviation

The problem here is to devise a

methodology that is capable of being

standardized.

Future work (continued)

Any suggestions for future work will be seriously

considered. However, the decision whether to

proceed will be made as a result of an

international ballot on a new work item proposal.

Success of the ballot depends on a scoring

system and whether at least 5 experts offer to

take part in the development of the new

standard.