Quality assurance for measuring equipment -

INTERNATIONAL STANDARD

IS0

10012-2

First edition 1997-09- 15

Quality assurance for measuring equipment -

Part 2:

Guidelines for control of measurement processes

Assurance de la qua/it6 des kquipements de mesure -

Partie 2: Lignes directrices pour la maftrise des processus de mesure

Reference number IS0 10012-2: 1997(E)

IS0

Contents

1 Scope . . . ..*... 1

2 Normative references . . . ..~...~... 1

3 Definitions . . . ...*...* 1

4 Recommendations . . . ...“.. 6

4.1 General . . . ...~... 6

4.2 Documentation . . . ..*...*...*...**... 6

4.3 Measurement processes . . . ..*....*... 7

4.4 Measurement process set-up and design . . . ..~... 7

4.5 Metrological confirmation system . . . 8

4.6 System for the control of measurement processes . . . ..~... 8

4.7 Data analysis for the control of measurement processes . . . ..*... 8

4.8 Surveillance of the measurement process . . . ..*... 9

4.9 Intervals of surveillance . . . ... 9

4.10 Indicated failure of a controlled measurement process . . . ..*...*... 9

4.11 Verification of the measurement process .,... 10

4.12 Identification of verified measurement processes . . . ..*... 10

4.13 Records of the control of measurement processes . . . ..L... 10

4.14 Personnel . . . ..*... 10

4.15 Periodic audit and review of the system of control of measurement processes . . . 11

Annex A Overview . . . ..‘... 12

A.1 General . . . ... 12

A.2 Using check standards . . . ..*... 14

0 IS0 1997

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the publisher.

International Organization for Standardization Case postale 56 ^l CH-1211 Geneve 20 ^l Switzerland Internet central @I iso.ch

x.400 c=ch; a=400net; p=iso; o=isocs; s=central Printed in Switzerland

0 IS0

IS0 10012=2:1997(E)

A.3 Frequency of monitoring of measurement processes ... 15 A.4 Monitoring the control limits ... 15 Annex B Bibliography ... 16

. . . III

IS0 10012=2:1997(E)

Foreword

IS0 (the International Organization for Standardization) is a worldwide federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carried out through IS0 technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. IS0 collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.

Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.

International Standard IS0 10112-2 was prepared by Technical Committee ISOTTC 176, Quality management and quality assurance, Subcommittee SC 3, Supporting technologies.

IS0 10012 consists of the following parts, under the general title Quality assurance for measuring equipment:

- Par? 1: Metrological confirmation system requirements for measuring equipment Part 2: Guidelines for control of measurement processes

Part 1 (now under revision) was published under the title Qualify assurance requirements for measuring equipment - Par? 1: Metrological confirmation system for measuring equipment.

Annexes A and B of this part of IS0 10012 are for information only.

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IS0 10012-2:1997(E)

Introduction

IS0 10012 is part of the IS0 9000 family of standards.

This part of IS0 10012 is intended to be used as a guidance document for quality management or as a requirement document by agreement between the Supplier and the Customer. It is written in the context of a Customer and a Supplier, both terms being interpreted in the broadest sense. The “Supplier” may be a manufacturer, an installer or a service organization. The “Customer” may be a procurement authority or a customer using a product. Suppliers become customers when procuring products from vendors or other outside sources.

Reference to this International Standard is recommended to be made:

- by a Customer when specifying products required;

by a Supplier when specifying products offered;

by consumer or employee interests, or by legislative or regulatory bodies;

in assessment and audit of the control of measurement systems.

This part of IS0 10012 includes (in clause 4) both recommendations and guidance. It is written in the context of a Customer and a Supplier for a product, all terms being interpreted in the broadest sense given in IS0 8402.

In order to distinguish clearly between recommendations and guidance, in clause 4 the latter appears in italic type- face, in a box, after each corresponding paragraph under the heading “GUIDANCE”.

The text under “GUIDANCE” is for information only and contains no requirements. Statements given there are not to be construed as adding to, limiting, or modifying any requirement.

Part 1 of IS0 10012 contains general quality assurance requirements for the control of measuring equipment. Part 2 provides supplementary guidance on the application statistical process control when this is appropriate for achieving the objectives of part 1.

Measurement should be considered as an overall process. The methods for the control of measurement processes, based on the regular monitoring and analysis of measurement data, are applicable at all levels of measurement, ranging from the calibration of the Supplier’s measurement standards by an outside metrology laboratory to the Supplier’s own routine measurements. Procedures for the control of measurement processes can be used

to detect unusual variations in the operation of the measurement process;

- to detect problems with repeatability;

to identify and quantify the compensations or correction factors for any drift;

to help in the identification of predictable periodic variations, including cyclic variations;

to provide some of the documentation required by quality assurance requirements.

This concept of “Measurement Process Control” has also been known as ‘LMeasurement Assurance”.

In practice, the control of measurement processes is specifically applicable to critical or complex measurement systems (for example for safety or economic purposes). A Supplier may consider that the confirmation system specified in IS0 10012-I provides adequate control for routine processes, such as the testing of non-critical components.

IS0 10012=2:1997(E)

The Metrological Confirmation System described in IS0 10012~1 is intended to ensure that the measurements (performed using measuring equipment that is within its confirmation interval) are sufficiently accurate for the purpose. However, while the confirmation interval, based on experience, provides a high probability that measuring equipment is still functioning correctly at the expiry of its confirmation interval, it cannot guard against random failure or unsuspected and not easily visible damage. Additionally, the metrological confirmation system does not provide any assurance that the measuring equipment is being used correctly. Even the most accurate measuring equipment will provide incorrect measurement results when used incorrectly. Correctly written measurement procedures should be a safeguard, but it is not always possible to ensure that the procedures are being correctly followed.

Controlling measurements as processes, in accordance with part 2 of IS0 10012, reduces the possibilities of problems arising from random failure, damage or misuse. The effectiveness or degree of such reductions is determined by how frequently the checks (process controls) are undertaken. The frequency is a matter of managerial and commercial judgement and, therefore, specific (quantified) recommendations concerning the frequency will not be made in this part of IS0 10012.

Measuring equipment is only one of many factors affecting measurements. The concept of a “Measurement process” views measurement as a complete process starting from analysis of the implications of the scientific basis of the measurement, traceability of the values of measurement standards, calibration and, if necessary, adjustment through verification and metrological confirmation, to the results produced by the measuring equipment at the place of work and under the conditions of use.

The operation of a metrological confirmation system often involves measuring equipment having to be taken from its place of use to a central metrological laboratory for calibration, adjustment or repair and, if necessary, verification and re-confirmation. It is frequently found that such returned equipment is, in fact, operating correctly, no repairs or adjustments being necessary. Indeed, if this were not so for a high proportion of the equipment, there would be a significant chance that incorrect results had been obtained while it was in use, especially towards the end of its confirmation interval. If measuring equipment, returned for confirmation merely because it has reached the end of its confirmation interval, is found to be functioning correctly, it is possible to argue that, with hindsight, it could have been left in use, at a great saving in cost and inconvenience. The risks of producing erroneous measurement results will usually dictate that this argument cannot be accepted.

The combination of metrological confirmation and control of measurement processes that is considered to be adequate depends on economic aspects, safety, fitness for use and other factors.

An overview annex A.

of the concept of viewing the control of instruments and equipment as continuous process es is given in

A list of informative references is given in annex B.

INTERNATIONAL STANDARD 0

IS0 10012-2:1997(E)

Quality assurance for measuring equipment - Part 2:

Guidelines for control of measurement processes

1 Scope

1.1 This part of IS0 10012 contains quality assurance recommendations that may be used by a Supplier to provide enhanced assurance that measurements are made with the intended accuracy. It also contains guidance on the implementation of the recommendations.

1.2 It is also intended to be used as a guide for quality management or as a requirement document on agreement between the Supplier and the Customer.

1.3 This part of IS0 10012 is applicable to measurement processes. It deals with elements that may affect measurement results, such as measurement procedures, personnel, etc., not extensively covered in IS0 10012-I.

1.4 This part of IS0 10012 is applicable to

- organizations where measurement is used to demonstrate compliance with specified requirements;

- suppliers of products who operate a quality system in which measurement results are used to demonstrate compliance with specified requirements; this includes operating systems that meet the requirements of IS0 9001, IS0 9002 and IS0 9003.

2 Normative references

The following standards contain provisions which, through reference in this text, constitute provisions of this part of IS0 10012. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this part of IS0 10012 are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and IS0 maintain registers of currently valid International Standards.

IS0 8402: 1994, Quality management and quality assurance - Vocabulary.

IS0 10012-l :I 992, Quality assurance requirements for measuring equipment - Part I: Metrological confirmation system for measuring equipment,

3 Definitions

For the purposes of this part of IS0 10012, the definitions given in IS0 8402 and the following apply. Most of the definitions are taken from the International vocabulary of basic and general terms in metrology (VIM). Some are taken from IS0 10012-I. Relevant reference numbers are given in square brackets following the definitions. These definitions are included to assist the understanding of the concepts used in this part of IS0 10012 without the need to consult too many other documents.

1

IS0 10012-2:1997(E)

31

accuracy of measurement

closeness of the agreement between the result of a measurement and a true value of the measurand NOTES

“Accuracy” is a qualitative concept.

2 The term precision should not be used for “accuracy”.

[VIM: 1993, 3.51 32

abjustment (of a measuring instrument)

operation of bringing a measuring instrument into a state of performance suitable for its use NOTE Adjustment may be automatic, semiautomatic or manual.

[VIM:1 993, 4.301 33

cilibration

set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and the corresponding values realized by standards

NOTES

1 The result of a calibration permits either the assignment of values of measurands to the indications or the determination of corrections with respect to indications.

2 A calibration may also determine other metrological properties such as the effect of influence quantities.

3 The result of a calibration may be recorded in a document, sometimes called a calibration certificate or a calibration report.

[VIM:1 993, 6.1 I]

34

check standard

measuring equipment, product, or other objects serving to collect a data base for the control of a measurement process, by being measured by that process

NOTES

1 See also VIM:1 993, 6.7, note 2.

2 A check standard should only be used as a check standard.

3 An overview of the use of check standards is given in annex A.

35

cbntrol of measurement processes

monitoring and analysis of data from a measurement process, together with corrective actions, intended to maintain the process of measurement continuously within a specification

NOTE This may include the use of check standards, control charts, or their equivalents.

36

limits of permissible error (of a measuring instrument) maximum permissible errors (of a measuring instrument)

extreme values of error permitted by specifications, regulations, etc. for a given measuring instrument [VIM:1 993, 5.211

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3.7 measurand

particular quantity subject to measurement

EXAMPLE Vapour pressure of a given sample of water at 20 “C.

NOTE The specification of a measurand may require statements about quantities such as time, temperature and pressure.

[VIM:1 993, 2.61 3.8

measurement

set of operations having the object of determining the value of a quantity NOTE The operations may be performed automatically.

[VIM:1 993, 2.11 39

measurement procedure

set of operations, described specifically, used in the performance of particular measurements according to a given method

NOTE A measurement procedure is usually recorded in a document that is sometimes itself called a “measurement procedure” (or a measurement method) and is usually in sufficient detail to enable an operator to carry out a measurement without additional information.

[VIM:1 993, 2.51 3.10

measurement process

set of interrelated resources, activities, and influences which produce a measurement NOTES

1 The resources concerned include measuring equipment, measurement procedures, operator.

2 “Influences” are all factors such as those caused by which add to the variability or the bias of the process.

the environment which may or may not be cant rolled or controllable and

3 See also definition 1.2 (process) of IS0 84027 994.

4 A measurement process may consist of measurements made, for example, by:

a)

general purpose measuring equipment in a general plant environment, using informal

b) trained calibration laboratory technicians using a measurement system with a temperature controlled oil bath, reference standard resistors, comparators, and other auxiliary equipment, following a detailed procedure for the purpose of calibrating other standard resistors;

Cl

5 A measurement process may consist of the use of a single measuring instrument.

3.11

measuring equipment

all of the measuring instruments, measurement standards, reference materials, auxiliary apparatus and instructions that are necessary to carry out a measurement

NOTES

This term includes measuring equipment used in the course of testing and inspection, as well as that used in calibration.

3

IS0 10012-2:1997(E)

2 I n the context of this part of IS0 1001

2,

and “measurement standards”. More over, a “reference material” is considered to be a type of “measurement standard”.

[Adapted from IS0 10012-I :I 992, 3.21 3.12

measuring instrument

device intended to be used to make measurements, alone or in conjunction with supplementary device(s) [VIM:1 993, 4.11

3.13

metrological confirmation

set of operations required to ensure that an item of measuring equipment is in a state of compliance with requirements for its intended use

NOTES

1 Metrological confirmation normally includes, inter alia, recalibration, as well as any required sealing and labelling.

calibration, any necessary adjustment or repair and subsequent

2 For brevity, in this part of IS0 10012, this term is referred to as “confirmation”.

[ISO 10012-1 :I 992, 3.11 3.14

quality audit

systematic and independent examination to determine whether quality activities and related results comply with planned arrangements and whether these arrangements are implemented effectively and are suitable to achieve objectives

NOTES

1 The quality audit typically applies to, but is not limited to, a quality system or elements thereof, to processes, to products or to services. Such audits are often called “quality system audit”, “process quality audit”, “product quality audit” or “service quality audit”.

2 Quality audits cooperation with

are carried the relevant

out by staff personnel.

not having direct responsibility in the areas being audited but, preferably, working

3 One purpose of a quality audit is to evaluate the need for improvement or corrective action. An audit should not be with quality surveillance or inspection activities performed for the purpose of process control or product acceptance.

in

confused

4 Quality audits can be conducted for internal or external purposes.

[ISO 8402:1994, 4.91 3.15

resolution (of a displaying device)

smallest difference between indications of a displaying device that can be meaningfully distinguished NOTES

For a digital displaying device, this is the change in the indication when the least significant digit changes by one step.

2 This concept also applies to a recording device.

[VIM:1 993, 5.121

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IS0 10012-2: 1997(E)

3.16 stability

ability of a measuring instrument to maintain constant its metrological characteristics with time NOTES

Where stability with respect to a quantity other than time is considered, this should be stated explicitly.

2 Stability may be quantified in several ways, for example:

- in terms of the time over which a metrological characteristic changes by a stated amount, or - in terms of the change in a characteristic over a stated time.

[VIM:1 993, 5.141 3.17

traceability

property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties

NOTES

The concept is often expressed by the adjective “traceable”.

2 The unbroken chain of comparisons is called a “traceability chain”.

[VIM:1 993, 6.101 3.18

uncertainty of measurement

parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand

NOTES

1 The parameter may be, for example, a standard deviation (or a stated I evel of con fidence.

given multiple of it), or the half-width of an interval having a

2 Uncertainty of measurement comprises, in general, many components. Some of these components may be evaluated from the statistical distribution of the results of series of measurements and can be characterized by experimental standard deviations. The other components, which can also be characterized by standard deviations, are evaluated from assumed probability distributions based on experience or other information.

3 It is understood that the result of the measurement is the best estimate of the value of the measurand, and that all components of uncertainty, including those arising from systematic effects, such as components associated with corrections and reference standards, contribute to the dispersion.

[VIM:1 993, 3.91

4 This definition is identical with that in the Guide to the expression of uncerfainty in measurement, in which its rationale is detailed (see, in particular, 2.2.4 and annex D).

3.19 verification

confirmation by examination and provision of objective evidence that specified requirements have been fulfilled NOTES

1 In design a .nd development, verification concerns the confo rmity with the input requirement for that activity.

process of examining the result of a given activity to determine 2 The term “verified” is used to designate the corresponding status.

[ISO 8402:1994, 2.171

IS0 10012=2:1997(E)

4 Recommendations 4.1 General

The Supplier should establish and document objective performance criteria and procedures for measurement processes and the control of measurement processes. They should be specific in terms of which measurement processes are subject to the provisions of this part of IS0 10012.

The Supplier should ensure that all procedures are adequate for their purpose. In particular, procedures should contain sufficient information to ensure their proper implementation, to ensure consistency of application from one application to another, and to ensure valid measurement results.

The procedures should provide for the prompt detection of deviations exceeding the stated limits of permissible deviation, and for their timely correction.

The procedures should be available, as necessary, to staff involved in performing measurements and the control of measurement processes.

GUIDANCE

Procedures may be, but are not necessarily, limited to the compilation of published standard measurement process control practices. The amount of detail in procedures should be commensurate with the complexity of the measurements and the control of the measurement processes.

It is not economical for the supplier and customer to control all measurement processes equally. The supplier and customer should examine complex measurement processes, and agree which par?(s) of the process are critical and suitable for the application of the requirements of this par? of IS0 100 12.

4.2 Documentation

The Supplier should document the methods used to implement each recommendation of this part of IS0 10012.

This documentation should be an integral part of the Supplier’s quality system. It should be specific in terms of which items of equipment (including computer hardware and software) are included in each measurement process.

The documentation should state the allocation of responsibilities and the actions to be taken. The required performance of the measurement process should be documented.

GUIDANCE

Examples of documentation may include:

- specification of measuring instruments;

- measurement procedures;

- instructions for the operator,.

- validation reports;

- verification repotis;

- measurement uncertainty budgets;

- limits of permissible error;

- details of and/or listings of computer programs used.

NOTE “Documentation” is used in the widest possible sense to cover all means of recording and presenting information.

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4.3 Measurement processes

The performance characteristics required for the intended use of the measurement process should be characterized.

GUIDANCE

Examples of the characteristics for instruments, equipment or methods include:

- uncertainty in use;

- stability;

- range;

- resolution;

- repeatability;

- reproducibility;

- skill level of operator.

Other characteristics may be important for some measurement processes.

Measurement processes and documentation should be maintained to take account of any corrections, conditions of use (including environmental conditions), etc. that are necessary to achieve the required performance.

The Supplier should fully specify and document each measurement process that is to be subject to control. The complete specification of such a measurement process should include identification of all relevant equipment, measurement procedures, measurement software, conditions of use, operator capabilities and all other factors affecting the reliability of the measurement.

GlJlDA NCE

A measurement process may be limited to the use of a single measuring instrument.

It is intended that this part of IS0 100 12 will be applied to the control of measurement processes primarily by the use of check standards.

4.4 Measurement process set-up and design

The Supplier should determine the limits of uncertainty and functional requirements for measurement processes, based on the needs of the Customer. The measurement processes designed to meet these needs should be fully documented and, if necessary, agreed with the Customer. This is particularly important for measurement processes new to the Supplier and/or where there are insufficient relevant process data available.

GUIDANCE

All influence quantities affecting the achievement of the requirements of the measurement process should be identified. The impact of influence quantities on the measurement process should be quantified. It may be necessary to design and carry out specific experiments or investigations to do this. When this is not possible, the data, specifications and warnings provided by an instrument (or equipment) manufacturer should be used.

IS0 10012=2:1997(E)

4.5 Metrological confirmation system

All measuring equipment essential for the correct performance of a measurement process should be confirmed in accordance with IS0 10012-I.

The information collected for the control of the measurement process by using check standards may assist in the determination of calibration intervals as required by IS0 10012-l.

4.6 System for control of measurement processes

The Supplier should implement a system for the control of the measurement process that is adequate to assure that the performance of the measurement process is within the required limits of uncertainty. This system should take account of any stated and acceptable risk of failure to comply with specified requirements. The Supplier should be able to demonstrate to the Customer that the required performance is achieved.

When taking decisions based on data arising from measurement process control systems, the Supplier should follow documented procedures.

4.7 Data analysis for control of measurement processes

For each measurement process that is subject to control, the Supplier should identify measurement process elements for analysis, and should establish appropriate limits for these elements.

GlJlDA NCE

For example: For the process of measuring impedance, resistance may be chosen as an element for analysis.

The choice of elements and control limits should be such as to maintain the measurement accuracies required, commensurate with the risk of failure to comply with specified requirements. These may include the effects of operators, equipment, ambient conditions, influence quantities, application method, etc.

The Supplier should document sources of known measurement variability that are not included in the specified process, and should document how the uncertainty contributions are accounted for in measurement uncertainty.

GUIDANCE

Different sets of specified process variables may result in measurement processes that differ at a specified level of significance. (See IS0 5725.) The existence or absence of such statistically significant differences should be documented. This documentation may form pat? of the overall measurement uncertainty budget for the process.

EXAMPLES

I The variability or drift of a working standard can be quantified by means such as trend charts or their equivalents.

2 A different mix of personnel operating the measurement process or a different range of environmental conditions may constitute a process that differs at the specified level of significance.

4.8 Surveillance of the measurement process

The control of measurement processes should be carried out in accordance with documented procedures, applying generally accepted process control principles.

The controlled measurement process should be monitored at one or more values of the measurand. These values should be appropriate for the range of values in which the process is used.

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The results obtained should be recorded.

IS0 10012-2: 1997(E)

GUIDANCE

The use of check standards with control charts can objectively show compliance with this recommendation.

Control charfs demonstrate the actual performance and thus facilitate auditing, but they can readily be replaced by equivalent numerical methods. Other methods of surveillance are acceptable only if they are based on generally accepted process control principles.

A series of measurements of a check standard can be analysed using a control char? or equivalent means to judge whether or not a given measurement process is in a state of control.

NOTE The use of check standards and control charts may not identify ail errors that may arise in or from the measurement process.

4.9 Intervals of surveillance

The Supplier should document, by procedure or otherwise, the intervals at which the process is to be checked.

GUIDANCE

Initially, the frequency of checking should be high but, as confidence in the measurement process is gained, the frequency may be reduced if this is shown to be appropriate. (See IS0 8258: 199 I, 10.4.)

4.10 Indicated failure of a controlled measurement process

When a relevant measurement process parameter is found to exceed specified limits, or when a sequence of checks shows an unacceptable pattern, the Supplier should either take corrective action to bring the measurement process back under control, or confirm that it remains under control.

The Supplier should document the criteria for taking corrective action.

When the measurement process control system indicates a failure to meet the stated measurement process requirements, the Supplier should take the necessary corrective actions. The Supplier should document these actions.

GUIDANCE

Corrective actions to be taken when programme performance fails to meet stated objectives may, for example, consist of:

- decreasing intervals between checks of one or more processes or of the instruments used in one or more processes,*

- repairing or eliminating unstable or unreliable instruments;

- extending the time span over which the measurements are made;

- decreasing the uncertainty acceptable for the measuring equipment at the time of confirmation (i.e. increase the accuracy required for the measuring equipment);

- increasing the number of influence quantities being checked;

- upgrading the operator competence.

IS0 10012=2:1997(E)

4.11 Verification of the measurement process

The results of the surveillance of the measurement process and any resulting corrective actions should be documented by the Supplier to demonstrate that the measurement process has continuously complied with the agreed (stated) requirements for the measurement process which is the subject of verification.

The verification documentation for each measurement process should ensure traceability, within the stated uncertainty.

GUIDANCE

For traceability to applicable measurement standards, see IS0 10012-1:1992, 4.15.

4.12 Identification of verified measurement processes

The elements of a verified measurement process should be clearly identified, individually or collectively.

Instruments or equipment confirmed for use only in a particular measurement process or processes should be clearly labelled or otherwise identified to indicate this. This is particularly important when instruments used for production control are also used for check standard measurements.

4.13 Records of control of measurement processes

The Supplier should maintain sufficient records to demonstrate compliance with the requirements of the measurement process control system, including:

- a full description of the measurement process control system implemented, including all unique elements, e.g.

operators, any unique measuring equipment or check standards used and the relevant operating conditions;

GlJlDA NCE

Batch identification may be adequate for consumable items used in measurement process control.

- the relevant data obtained from the measurement process control system, including any information relevant to the measurement uncertainty;

- any actions taken as a result of measurement process control data;

- the date(s) on which each measurement process control activity in the Supplier’s surveillance and verification programme was carried out;

- identification of any relevant verification and other documents;

- identification of the person responsible for providing the information for the records;

- qualifications (required and achieved) of the personnel.

The Supplier should maintain clear documented procedures on the retention (including the duration) and safeguarding of the records. Retention times for records should be established and recorded.

4.14 Personnel

The Supplier should ensure that all measurement process controls are performed by personnel having appropriate qualifications, training, experience, aptitude and supervision. The Supplier should specify the appropriate qualifications, training, experience, aptitude and supervision for the personnel in measurement process control and ensure that the personnel comply with these specifications.

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4.15 Periodic audit and review of the system of control of measurement processes

The Supplier should carry out, or arrange to be carried out, periodic and systematic quality auditing of the measurement process control system to ensure its continuing effective implementation and compliance with the requirements of this part of IS0 10012. Based on the results of the quality audits of the measurement process control system and of other relevant factors, such as comments from Customers, the Supplier should review and modify the measurement process control system as necessary.

Plans and procedures for the quality audit and review of the measurement process control system should be documented. The conduct of the quality audit and review and any subsequent corrective actions should be recorded.

GUIDANCE

Failure of the measurement process control system due, for example, to deterioration of the check standard or change of operator competence, may be revealed by post-process indications such as:

- analysis of control charts;

- subsequent inspections,*

- interlaboratory comparisons;

- customer complaint.

11

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Annex A (informative)

Overview

A.1 General

This annex is intended to give a brief overview of controlling measurements as continuing processes rather than as individual events. It concentrates on describing the use of check standards. As the control of measurement processes is a relatively new approach to controlling measurements, a bibliography is given in annex B. These references, while not exhaustive, describe the underlying rationale of the control of measurement processes, the problems it helps to resolve and the opportunities it presents for enhancing product quality.

Up to the present time, the largest amount of literature on the control of measurement processes has been written with reference to ‘LMeasurement Assurance”.

To assist the user of this part of IS0 10012 to understand some of the concept presented in this annex, the following additional definitions are presented.

A.1 .I

bias (of a measuring instrument)

systematic error of the indication of a measuring instrument NOTE The bias of a measuring instrument is normally number of repeated measurements.

estimated _bY averaging the error of indication over an appropriate

[VIM:1 993, 5.251 A.l.2

conventional true value (of a quantity)

value attributed to a particular quantity and accepted, sometimes by convention, as having an uncertainty appropriate for a given purpose

EXAMPLES a) at a given value;

location, the value assigned to the quantity realized by a reference standard may ^be taken as a conventional true

b) the CODATA (1986) recommended value for the Avogadro constant, NA: 6,022 136 7 x 1 023 mol-l.

[VIM:1 993, I.201 A.l.3

correction factor

numerical factor by which the uncorrected result of a measurement is multiplied to compensate for systematic error NOTE Since the systematic error cannot be known perfectly, the compensation cannot be complete.

[VIM:1 993, 3.161

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A.l.4

error (of indication) of a measuring instrument

indication of a measuring instrument minus a true value of the corresponding input quantity NOTES

Since a true value cannot be determined, in practice a conventional true value is used.

2 This concept applies mainly where the instrument is compared to a reference standard.

3 For a material measure, the indication is the value assigned to it.

[VIM:1 993, 5.20]

A.1.5

influence quantity

quantity that is not the measurand but that affects the result of the measurement EXAMPLES

4 b) Cl

temperature of a micrometer used to measure length;

frequency in the measurement of the amplitude of an alternating electric potential difference;

bilirubin concentration in the measurement of haemoglobin concentration in a sample of human blood plasma.

[VIM:1 993, 2.71 A.l.6

precision

the closeness of agreement between independent test results under stipulated conditions NOTES

Precision depends only on the distribution of random errors and does not relate to the true value or the specified value.

2 The measure of precision usually is expressed in terms of imprecisio results. Less precisi on is reflected by al arger standard deviation.

n and corn puted as a standard deviation of the test

3 “Independent test results” means results obtained in a manner not influenced by any previous result on the same or similar test object. Quantitative measures of precision depend critically on the stipulated conditions. Repeatability and reproducibility conditions are particular sets of extreme stipulated conditions.

[ISO 3534-l :I 993, 3.141 A.l.7

true value (of a quantity)

value consistent with the definition of a given particular quantity NOTES

This is a value that would be obtained by a perfect measurement.

2 True values are by nature indeterminate.

3 The indefinite article _{“a” 9} rather than the definite article “the”, is used many values consistent with the definition of a given particular quantity.

in conjunction with “true value” because there may be

[VIM:1 993, 1 .I 91

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A.2 Using check standards

A.21 A check standard is some device that is similar in kind to the items being measured by the measurement process that is to be controlled. By making regular measurements on a check standard that is expected to be stable and plotting the data on a control chart(s), one obtains the Type A evaluation of one or more of the contributions to the measurement uncertainty. When the average of the measurements of a check standard differs from the previously established value of that standard, one should suspect that an apparent bias has crept into the measurement process. This apparent bias should be tested to determine whether it is significant. Rigorous statistical tests are available for making such tests. Any bias should be removed or compensated for, as appropriate.

A.2.2 A check standard need not be a single object; for example, in specific and limited circumstances the difference between two standards may be used as a check standard. One of the results of this is that the definition of a check standard should be framed to include a wide range of possibilities. (See 3.4.)

A.2.3 A check standard should be measured or calibrated by a process that is independent of the process which it is used to control and which is also more precise than the process it controls. However, even without such independence and/or precision, the check standard can nevertheless serve a useful function to control the process.

Traceability of the independent measurement or calibration of the check standard is essential if the results obtained are to be used to assess the bias in the measurement process being controlled, and to assess the uncertainty contribution due to any corrections applied for this.

A.2.4 Plotting the data simply as obtained, or more formally as a control chart, can identify unexpected temporary biases or process variabilities as well as long-term trends. If the measured value of the check standard changes with time, then one should conclude that either the check standard is changing or that the measurement process is changing, or both. In such cases, confidence in the measurement process should be suspended until the reason for the variation is found and statistical control can again be asserted.

A.25 It should be noted that one may achieve statistical control of a time dependent when the time-dependent behaviour is predictable.

If one measurement on the check standard indicates a possible out-of-control condition, then the measurement should be repeated. The formalized control chart methods lay down specific rules for dealing with single measurements outside specified limits. (See also A.4.)

A confirmed out-of-control condition should be identified and corrected.

A.2.6 There are many different types of control charts for variables that can be applied to controlling measurement processes. Average (x) and range (R) charts are particularly useful. The output of a process is under control only when its mean (Xchart) and its variability (R-chart) are under control. This will be so when the values plotted remain within the limits set, with valid statistical reason for any points (outliers) outside these limits. When this is not so, a changed measurement process is indicated and the reasons for this should be investigated.

A.2.7 Some Suppliers prefer the use of cumulated sum (CUSUM) charts instead of x’- and R-charts. CUSUM charts are often better suited to detecting small changes with greater confidence that the measurement process is under control.

A.2.8 Sometimes, sample standard deviations, S, are used instead of ranges resulting in s-charts. R-charts, however, are the preferred measures of process variability since they are better than standard deviations as estimators of process variability when the samples are small. Statistically, standard deviations begin to become better when more than about ten observations are available. Usually, however, the number of observations has to be kept small because the time and cost of obtaining them.

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A.3 Frequency of monitoring of measurement processes

A.3.1 Care should be taken to ensure that the frequency of check standard measurements is appropriate for the measurement process. The underlying effects of time on the measurement process should be fully investigated and understood. Several hours to several days are sometimes required to obtain the necessary observations for one check measurement. Shorter time periods may cause the emergence of significant “between-group” variability which prevents the establishment of statistical control.

A.3.2 The frequency at which check standard measurements are made depends on four main factors:

- the amount of control;

the degree of assurance required;

the degree of criticality of the measurement uncertainty;

- the stability of the process.

Normally one would re-measure the check standard frequently for a new process and/or for a new check standard.

Once experience is gained and the process is shown to be stable and to remain in statistical control, the intervals between check standard measurements can often be lengthened.

A.3.3 Check standard measurements and their frequency should, of course, be specified by procedure, like all other important requirements of the measurement process. Well-established measurement processes with a history of few assignable causes may be deemed to require fewer check standard measurements than other measurement processes. It may be sufficient to make a measurement on a check standard at randomly selected times during a working day or week. If check standard measurements are not made with each measurement of unknown measurands, one should ensure that such measurements are made at random times. A measurement that is always made at the same time of day, for example, may mask between-group random errors. The same may be true for any other condition that is held constant only for the check standard measurement.

A.3.4 Some measurement processes, however, are not sufficiently critical to warrant check standard measurements and the maintenance of control charts.

A.4 Monitoring the control limits

All data obtained from the use of check standards should be computed and plotted promptly. The resultant charts should be regularly monitored and assessed by an independent person (i.e. not the person carrying out the measurements). When monitoring the results obtained from the use of check standards, it is important that all data are considered. Such monitoring and assessment should be realistic. For example, if the uncertainty limits assigned to the measurement process are too narrow, indications of an increased process variability may be frequent.

However, for a well-established process, it may be better to investigate if a process has changed, rather than re- computing the uncertainty limits and making them wider. With control limits set at + 2 times the standard deviation (usually called “warning limits”), “outliers” in the range of two to three standard deviations are statistically predicted from time to time. The occurrence of such a point does not necessarily mean that the process is out of control. The normal procedure would be to repeat the check standard measurement when an outlier is observed. If subsequent points fall within the warning limits, the process is still in control. Periodically, the frequency of occurrence of outliers should be investigated, and possible causes identified. If frequency of outliers from unidentified causes indicates that they are likely to occur more often than would be predicted statistically, the process has changed and should be re-assessed.

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Annex B (informative) Bibliography

PI PI PI WI PI PI VI PI PI [I 01 [I 11 WI WI [I 41 Cl51 P61 WI hl81

IS0 3534-l :I 993, Statistics - Vocabulary and symbols - Part I: Probability and general statistical terms.

IS0 3534-2:1993, Statistics - Vocabulary and symbols - Part 2: Statistical quality control.

I SO 3534-3:-l ), Statistics - Vocabulary and symbols - Part 3: Design of experiments.

IS0 5725-l :I 994, Accuracy (trueness and precision) of measurement methods and results - Part I: General principles and definitions.

IS0 5725-2:1994, Accuracy (trueness and precision) of measurement methods and results - Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method.

IS0 5725-3:1994, Accuracy (trueness and precision) of measurement methods and results - Part 3:

Intermediate measures of the precision of a standard measurement method.

IS0 5725-4:1994, Accuracy (trueness and precision) of measurement methods and results - Part 4: Basic methods for the determination of the trueness of a standard measurement method.

IS0 5725-5: -*I Accuracy (trueness and precision) of measurement methods and results - Part 5:

Alternative methods for the determination of the precision of a standard measurement method.

IS0 5725-6:1994, Accuracy (trueness and precision) of measurement methods and results - Part 6: Use in practice of accuracy values.

IS0 7870:1993, Control charts - General guide and introduction.

IS0 787311993, Control charts for arithmetic average with warning limits.

IS0 7966: 1993, Acceptance control charts.

IS0 8258:1991, Shewhart control charts.

IS0 9000-I :I 994, Quality management and quality assurance standards - Part I: Guidelines for selection and use.

IS0 9001:1994, Quality systems - Model for quality assurance in design/development, production, installation and servicing.

IS0 9002: 1994, Quality systems - Model for quality assurance in production, installation and servicing.

IS0 9003: 1994, Quality systems - Model for quality assurance in final inspection and test.

IS0 9004-I :I 994, Quality management and quality system elements - Part I: Guidelines.

‘1 To be published. (Revision of IS0 35343:I 985)

*) To be published.

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[191 WI PV [=I WI PI

WI WI WI

WI WI WI WI

IS0 1001 I-1 :I 990, Guidelines for auditing quality systems - Part 1: Auditing.

IS0 Guide 30: 1992, Terms and definitions used in connection with reference materials.

IS0 Guide 33:1989; Uses of certified reference materials.

VIM: International vocabulary of basic and general terms used in metrology, BIPM/IEC/IFCC/lS0/0IML/IUPAC/IUPAP, 1993.

GUM: Guide to the expression of uncertainty in measurement, BIPM/IEC/IFCC/ISO/OIMUIUPAC/IUPAP, 1995.

EISENHART, C. Realistic Evaluation of the Precision and Accuracy of Instrument Calibration Systems. Journal of Research of the National Bureau of Standards - C. Engineering and Instrumentation, 67C, No. 2 (April-June) 1963.

SHEWHART, W. A. Economic Control of the Quality of Manufactured Product. Van Nostrand Company, New Jersey, 1931 (Re-published by ASQC, Quality Press, Milwaukee, WI, USA ,198O).

CAMERON, J. M. Measurement Assurance. NBS/R 77 - 1240. National Bureau of Standards, Washington, DC, USA, April 1977.

ASTM Manual on Presentation of Data and Control Chart Analysis: 6th edition: ASTM Manual Series: MNL 7 - Revision of Special Technical Publication (STP) 75D. American Society for Testing and Materials, Philadelphia, 1990.

OIML International Document No. 16, Principles of assurance of metrological control.

DAVID, H. A. Order statistics. John Wiley, New York, 1981.

KOTZ, S., JOHNSON, N. L. (eds.) Encyclopaedia of Statistica/ Sciences. Wiley Interscience, New York, Vol. 2 (1982) and Vol. 7 (1986).

OAKLAND, J. S., FOLLOWELL, R. F. Statistica/ Process Control. 2nd edition, Heinemann Newnes, London, 1990.

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ICS 03.120.10; 17.020

Descriptors: measuring instruments, quality assurance, specifications, measurement, measuring systems, control procedures.

Price based on 17 pages