Terms and definitions used in connection with reference materials
second edition 1992
Foreword
ISO (the International Organinization for Standardizationl is a worldwide federation of national standards bodies (ISO menber bodies).The work of preparing International Standards is normally carried out through ISO 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. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. ISO Guides are intended essentially for internal use in ISO committees or in some cases for the guidance of membe bodies when dealing with matters that would not normally be the subject of an International Standard. ISO Guide 30 was drawn up by the ISO Committee on reference materials (REMCO). which is concerned with guidelines for the preparation, certification and use of reference materials (RMs) and certified reference materials (CRMs). The first edition of this Guide (1981) was the outcome of collaboration between REMCO and the organizations EEC, IAEA, OIML, IUPAC, IFCC and WHO, and was roduced largely by Dr D. A. Lowe of WHO and Prof. Dr R.
Neider of BAM. The revision leading to this second edition was undertaken because it had become apparent that some confusion existed as to what types of measurement
standardsor etalonsshould legitimately be included with in the definition of a reference material. Moreover, the recogination that certified reference materials are measurement standards made it desirable to examine the vocabulary of standards in metrology, as detailed in the International vocabulary of basic and general terms In
metrology (VIM), with particular reference to certified reference materials. Reference is made herein to the draft second edition of the VIM. When that second edition is
available in its final version, Guide 30 will be revised for harmonization with the VIM, if necessary. The same considerations apply to the use herein by REMCO of the drafts
of ISO 3534-1, ISO 3534-2, and ISO 5725-1.The first draft of this second edition of ISO Guide 30 was prepared by Dr A. J. Head (IUPAC) based on the replies to a questionnaire sent to all members of REMCO and liaison organizations. A revised version was accepted by the 15th meeting of REMCO (May 1991). Final approval was given by the 16th meeting of REMCO (October 1992).
Introduction
Reference materials (RMs) and certified reference materials (CRMs) (defined in 2.1 and 2.2) make possible the transfer of the values of measured or assigned quantities(physical, chemical, biological or technological) between one place and another. They are widely used for the calibration of measuring apparatus, for the evaluation of methods of analysis or test and for long-term quality assurance of measurements, and, in the case of certain biological and technological RMs, to enable the properties to be expressed conveniently in arbitrary units. All kinds of RMs and CRMs are playing an increasingly important role in national and international standardizing activities, in proficiency testing, and in the accreditation of laboratories This document is intended to serve as a guide to terms and definitions used in connection with reference materials and should prove useful in helping to ensure a greater degree of uniformity in the terminology used by different organizations concerned with the production and use of reference materials throughout the world.
1 Scope
This Guide recommends terms and the meanings that should be assigned to them when used in connection with reference materials, with particular attention to terms that are used in reference material certificates and corresponding certification reports.
2 Terms related to materials
2.1 reference material (RM): Material or substance one or more of whose property values are sufficiently homogeneous and well established to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials. NOTE A reference material may be in the form of a pure or mixed gas, liquid or solid. Examples are water for the calibration of viscometers, sapphire as a heat-capacity calibrant in calorimetry, and solutions used for calibration in chemical analysis.
2.2 certified reference material (CRM): Reference material, accompanied by a certificate, one or more of whose property values are certified by a procedure which establishes its traceability to an accurate realization of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty at a stated level of confidence.
NOTES
1 The definition of a reference material certificate is given in 4.2.
2 CRMs are generally prepared in batches for which the property values are determined within stated uncertainty limits by measurements on samples representative of the whole batch.
3 The certified properties of reference materials are sometimes conveniently and reliably realized when the material is incorporated into a specially fabricated device, e.g. a substance of known triple-point into a triple-point cell; a glass of known optical density into a transmission filter; spheres of uniform particle size mounted on a microscope slide. Such devices may also be considered as CRMs.
4 All CRMs lie within the definition of measurement standards or etalons given in the International vocabulary of basic and general terms in metrology (VIM)
5 Some RMs and CRMs have properties which, because they cannot be correlated with an established chemical structure or for other reasons, cannot be determined by exactly defined physical and chemical measurement methods. Such materials include certain biological materials such as vaccines to which an International unit has been assigned by the World Health Organization.
2.3 primary standard: Standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity, within a specified context.
NOTE The concept of primtary standard is equally valid for base units and derived uhits.
[VIM]
2.4 secondary standard: Standard whose value is as- signed by comparison with a primaty standard of the same quantity .
[VIM]
NOTE Most CRMs fall into this category since the certification of property values is usually carried out by a procedure traceable to primary standards. The position of a CRM in the measurement hierarchy is no indication of its suitability for a particular purpose. Thus, e,g, for the determination of trace metals in environmental
matrices, CRMs which are secondary standards, but which contain the metals in a similar state of chemical combination and in a similar matrix to the test sample, are greatly to be preferred over primary standards of pure metals. The analytical technique may be suffciently specialized to justity being designated as a distinct field of measurement in which the CRM could be considered as a primary standard.
2.5 charactarization: For a reference material, determination of one or more physical, chemical, biological, or techhological property values that are relevant to its intended end use.
2.6 homogeneity: Condition of being of uniform structure or composition with respect to one or more specified properties. A reference material is said to be homogeneous with respect to a specified property if the property value, as determined by tests on samples of specified size, is found to lie within the specified uncertainty limits, the samples being taken either from different supply units (bottles,
packages. etc.) or from a single supply unit.
2.7 stability: Ability of a reference material. when stored under specified conditions, to maintain a stated property value within specified limits for a specified period of time.
2.8 sample: Representative quantity of material extracted from a batch of reference material.
NOTES
1 The method of extracting the sample must ensure that it is representative of the batch with respect to the property or properties being investigated.
2 The term may be used to cover either a unit of supply or portion for analysis.
3 Terms related to measurement and testing
3.1 certified value: For a CRM, the value that appears in the certificate accompanying the material.
3.2 uncertified value: Value of a quantity, included in the certificate of a CRM or otherwise supplied. which is provided for information only but is not certified by the producer or the certifying body.
3.3 consensus value (of a given quantity): For a reference material, the value of the quantity obtained by interlaboratory testing, or by agreement between appropriate bodies or experts.
NOTE A consensus value could, through appropriate action by certifying body (see 4.4),become a certified value.
3.4 uncertainty of a certified value: Estimate attached to a certified value of a quantity which characterizes the range of values within which the "true value" is asserted to lie with a stated level of confidence.
NOTE See also VIM, definition 3.9 uncertainty of measurement.
3.5 precision: The closeness of agreement between independent test results obtained under prescribed conditions.
[ISO 5725-1]
3.6 accuracy: The closeness of agreement between a test result and the accepted reference value.
[ISO 5725-1]
NOTE See also VIM, definition 3.5 accuracy of measurement.
3.7 aceepted reference value: A value that serves as an agreed-upon reference for comparison and which is derived as:
a) a theoretical or established value, based on scientific principles;
b) an assigned value, based on experimental work of some national or international organization;
c) a consensus value,based on collaborative experimental work under the auspices of a scientific or engineering group.
[ISO 5725-1]
3.8 traceability: Property of the result of a measurement or the value of a standard whereby it can be relalted, with a stated uncertainty, to stated references, usually national or international standards, through an unbroken chain of comparisons.
NOTES
1 The concept is often expressed by the adjective traceable.
2 The unbroken chain of comparisons is called a traceability chain.
3 (Applicable only to the French text.)
4 Traceability of valuse in the certification of reference materials for chemical composition is discussed in ISO Guide 35:1989 (subclause 9.3.1 ) where attention is drawn to the special problems associated with chemicalaenalysis. Traceability of the chemical species is frequently of equal or greater importance than the traceability of the calibration of the instruments used in the analysis.
3.9 intetlaboratory test: Series of measurements of one or more quantities performed independently by a number of laboratories on samples of a given material.
NOTES
1 Other terms, including round robin test collaborative reference programme, and collaborative analytical study, are also used .
2 Interlaboratoty tests are carried out for many purposes other than the characterization of reference materials.
3.10 reference method: Thoroughly investigated method, clearly and exactly describing the necessary conditions and procedures, for the measurement of one or more property values that has been shown to have accuracy and precision commensurate with its intended use and that can therefore be used to assess the accuracy of other methods for the same measurement, particularly in permitting the characterization of an RM.
4 Terms related to certification and issuance of reference materials
4.1 certification of a refetence material: Procedure that establishes the value(s) of one or more properties of a material or substance by a process ensuring traceability to an accurate realization of the units in which the property values are expressed, and that leads to the issuance of a certificate .
4.2 reference material certificate: Document accompanying a certified reference material stating one or more property values and their uncertainties, and confirming that the necessary procedures have been carried out to ensure their validity and traceability.
NOTE See ISO Guide 31.
4.3 certification report: Document giving detailed information, supplementary to that contained in a certificate, e.g. the preparation of the material, methods of measurement, factors affecting accuracy, statistical treatment of results, and the way in which traceability was established.
4.4 certifying body: Technically competent body (organization or firm, public or private) that issues a reference material certificate which provides the information de-
tailed in ISO Guide 31.
NOTES
1 It may be the same as, or different from, the issuing body (i.e. the body from which the certified reference material is available) and the testing body (i.e. the organization that carried out the meaeurement leading to certification).
2 Unless or until nationally or internationally recognized accreditation procedures operate, the technical competence of a certifying body can only be judged on the basis of the information provided in reference material certificates and certification repone.
4.5 certified reference material producer: Technically competent body. (organization or firm public or private) that produces certified reference materials in accordance with the general and statistical principles detailed in ISO Guides 31 and 35.
Annex A
Additional terms
This annex lists some terms taken from the draft second edition (1992) of the International vocabulary of basic and general terms in metrology (VIM) and from the draft versions of ISO 3534-1 and ISO 3534-2 with, in some cases, notes added by REMCO. It is recommended that these terms be used in the field of RMs and CRMs with their same meanings.
A.1 (production) batch: A definite quantity of some commodity produced by one supplier at one time under conditions that are presumed uniform.
[ISO 3534-2]
NOTE When the commodity is a reference material, the uniform conditions of manufacture or production must be such as to ensure a homogeneous product.
A.2 value (of a quantity): Magnitude of a specific quantity generally expressed as a unit of measurement multiplied by a number.
[VIM]
A.3 true value for a quantity): Value perfectly consistent with the definition of a given specified quantity.
[VIM]
A.4 systematic error: Mean result of a large number of repeated measurements of the same measurand minus a true value of the measurand.
[VIM]
A.5 random error: Result of a measurement minus the mean result of a large number of repeated measurements of the same measurand.
[VIM]
A.6 level of confidence: The value 1—α of the probability associated with a confidence interval for range of uncertainty: see note 1 or a statistical tolerance interval (see ISO 3534-1 ),
NOTES
1 In statistics. the range of uncertainty is known as the confidence interval and the uncertainty limits are known as the confidence limits.
2 in simple, non-mathematical terms. the level of confidence may be defined as the percentage of the times when, on average, the "true value" of a property lies within the stated uncertainty.
A.7 repeatability (of results of measurements): Closeness of the agreement between the results of successive measurements of the same measurand carried out subject to all of the following conditions:
- the same measurement procedure;
- the same observer;
- the same measuring instrument, used under the same conditions;
- the same location:
- repetition over a short period of time.
[VIM]
A.8 reproducibility (of results of measurements): Closeness of the agreement between the results of measurements of the same measurand, where the measurements
are carried out under changed conditions such as:
- principle or method of measurement;
- observer;
- measuring instrument;
- location:
- conditions of use;
- time.
[VIM]
Annex B
Bibliography
[1] International vocabulary of basic and general terms in metrology (VIM) .
BIPM/IEC/ISO/OIML/IFCC/IUPAC/IUPAP.
Second edition. (To be published.)
[2] ISO 3534-1 :-Statistics - Vocabulary and symbols - Part 1: Probability and general statistical terms.(To be published.)
[3] ISO 3534-2:-. Statistics - Vocabulary and symbols - Part 2; Statistical quality control. (To be published.)
[4j ISO 5725-1 :- Accuracy (trueness and precision) of measurement methods and results - Part 1 General principles and definitions. (To be published.)
[5] ISO Guide 31 :1981, Contents of certificates of reference materials.
[6] ISo Guide 35: 1989, Certification of reference materials - General and statistical principles.
English alphabetical index
A
accepted reference value 3.7
accuracy 3.6
B
batch A.1
C
certification of a reference material 4.1
certification report 4.3
certified reference material (CRM) 2.2
certified reference material producer 4.5
certified value 3.1
certifying body 4.4
characterization 2.5
collaborative analytical study 3.9 (note 1)
collaborative reference programme 3.9 (note 1 )
consensus value (of a given quantity)3.3
H
homogeneity 2.6
interlaboratory test 3.9
L
level of confidence A.6
P
precision 3.5
primary standard 2.3
production batch A.1
R
random error A.5
reference material (RM) 2.1
reference material certificate A.2
reference method 3.10
repeatability (of results of measurements) A.7
reproducibility (of results of measurements) )A.8
round robin test 3.9 (note 1)
S
sample 2.8
secondary standard 2.4
stability 2.7
systematic error A.4
T
traceability 3.8
traceability chain 3.8 (note 2)
traceable 3.8 (note 1)
true value (of a quantity) A.3
U
uncertainty of a certified value 3.4
uncertified value 3.2
V
value (of a quantity) A.2