SIST-TS ISO/TS 20612:2010

TECHNICAL ISO/TS
SPECIFICATION 20612
First edition
2007-12-01

Water quality — Interlaboratory
comparisons for proficiency testing of
analytical chemistry laboratories
Qualité de l'eau — Comparaisons interlaboratoires pour des essais de
compétence de laboratoires de chimie analytique




Reference number
ISO/TS 20612:2007(E)
©
ISO 2007

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ISO/TS 20612:2007(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


COPYRIGHT PROTECTED DOCUMENT


©  ISO 2007
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 either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland

ii © ISO 2007 – All rights reserved

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ISO/TS 20612:2007(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope .1
2 Normative references .1
3 Terms and definitions .2
4 Symbols .2
5 Requirements relating to proficiency test provider .4
6 Participants .4
7 Proficiency test design .4
8 Execution of proficiency tests.6
9 Proficiency test evaluation .8
10 Presentation of results.15
11 Archiving and managing the results.15
Annex A (informative) Example of Q-method estimation principle.16
Annex B (informative) Determination of repeatability standard deviation, s .18
r
Annex C (informative) Example of evaluation method in Clause 9.19
Annex D (informative) Example of variance function calculation (9.3).21
Bibliography .25

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ISO/TS 20612:2007(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member 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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. 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.
In other circumstances, particularly when there is an urgent market requirement for such documents, a
technical committee may decide to publish other types of normative document:
In other circumstances, particularly when there is an urgent market requirement for such documents, a
technical committee may decide to publish other types of document:
⎯ an ISO Publicly Available Specification (ISO/PAS) represents an agreement between technical experts in
an ISO working group and is accepted for publication if it is approved by more than 50 % of the members
of the parent committee casting a vote;
⎯ an ISO Technical Specification (ISO/TS) represents an agreement between the members of a technical
committee and is accepted for publication if it is approved by 2/3 of the members of the committee casting
a vote.
An ISO/PAS or ISO/TS is reviewed after three years in order to decide whether it will be confirmed for a
further three years, revised to become an International Standard, or withdrawn. If the ISO/PAS or ISO/TS is
confirmed, it is reviewed again after a further three years, at which time it must either be transformed into an
International Standard or be withdrawn.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TS 20612 was prepared by Technical Committee ISO/TC 147, Water quality, Subcommittee SC 2,
Physical, chemical and biochemical methods.
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ISO/TS 20612:2007(E)
Introduction
Participation in interlaboratory tests in various test fields offers a testing laboratory an opportunity to obtain an
objective picture of its proficiency. Such tests serve as a confidence-building measure both for the laboratory
itself and for prospective clients.
This Technical Specification is based on the following international recognized documents:
⎯ ISO/IEC Guides 43-1 and 43-2;
⎯ ISO 13528;
⎯ The International Harmonized Protocol for the Proficiency Testing of Analytical Chemistry Laboratories –
(IUPAC, ISO, AOAC);
⎯ ILAC Guide 13;
⎯ ISO/IEC 17025;
⎯ ISO 5725-1 and ISO 5725-2.
As these documents only define a framework for design, execution and evaluation of proficiency testing by
interlaboratory comparisons, this Technical Specification describes in detail an evaluation procedure which is
especially suitable for the sector of water, waste water and sludge analysis, where results of interlaboratory
comparisons play an important role in the admission of laboratories to certain analytical tasks. Therefore, the
fairness of assessment of laboratories must be guaranteed. Assessment should not be dependent on the
provider, the date, or the method of evaluation.

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TECHNICAL SPECIFICATION ISO/TS 20612:2007(E)

Water quality — Interlaboratory comparisons for proficiency
testing of analytical chemistry laboratories
1 Scope
This Technical Specification specifies the criteria related to proficiency testing by interlaboratory comparisons
in the field of water, waste water and sludge analysis. In particular, it specifies the requirements in respect to
proficiency test providers and to the design, execution and evaluation of laboratory proficiency comparisons.
This document may be used if the determinands in the interlaboratory test may be regarded as capable of
measurement with a certain degree of continuity. This is generally the case for chemical constituents and
physicochemical determinands, but continuity does not always exist in the case of biological and/or
microbiological determinands.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 3534-1, Statistics — Vocabulary and symbols — Part 1: General statistical terms and terms used in
probability
ISO 5725-1, Accuracy (trueness and precision) of measurement methods and results — Part 1: General
principles and definitions
ISO 5725-1:1994/Cor.1:1998, Accuracy (trueness and precision) of measurement methods and results —
Part 1: General principles and definitions — Technical Corrigendum 1
ISO 5725-2, 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
ISO 13528, Statistical methods for use in proficiency testing by interlaboratory comparisons
ISO/IEC Guide 43-1, Proficiency testing by interlaboratory comparisons — Part 1: Development and operation
of proficiency testing schemes
ISO/IEC Guide 43-2, Proficiency testing by interlaboratory comparisons — Part 2: Selection and use of
proficiency testing schemes by laboratory accreditation bodies
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
ISO/IEC 17020, General criteria for the operation of various types of bodies performing inspection
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ISO/TS 20612:2007(E)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3534-1, ISO 5725-1, ISO 5725-2,
ISO/IEC Guide 43-1, ISO/IEC Guide 43-2 and the following apply.
3.1
breakdown point
smallest percentage of outlier laboratories above which the estimation method may be entirely inapplicable
3.2
efficiency
ratio of the variance of the optimum estimation method for normal distribution to the variance of the estimation
method under consideration, each assuming a normal distribution
NOTE This is expressed as a percentage.
3.3
sample
totality of a homogeneous analysis material with an identical composition or quality (similar to term batch)
3.4
subsample
defined portion of a sample obtained by suitable sample division and identical in terms of composition
4 Symbols
d Absolute difference between log-linear variance function and the logarithm of the reproducibility
i
standard deviation
G (x ) Generalized distribution function of interlaboratory differences with continuity correction (s )
1 i R
G (x ) Generalized distribution function of intralaboratory differences with continuity correction (s )
2 i r
g Quality limit
H (x ) Generalized distribution function of interlaboratory differences (s )
1 i R
H (x ) Generalized distribution function of intralaboratory differences (s )
2 i r
i
Index denoting the serial number of one of p samples
J Number of participants in the case of sample i
i
j Index denoting the serial number of one of J participating laboratories
k ,k Correction factors for calculating z -score
1 2 U
Overall mean
µ
Overall mean of i-th sample
µ
i
x Assigned value
a
n Number of measurements made by laboratory j
j
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ISO/TS 20612:2007(E)
q Quantile parameter
p
Number of samples (levels)
PG ,PG Testing values for testing variance function
0 1
s Reproducibility standard deviation
R
Reproducibility standard deviation of i-th sample
s
R
i
Reproducibility standard deviation for sample i adjusted using a variance function
sˆ
R
i
s Repeatability standard deviation
r
Standard deviation for proficiency assessment
σˆ
W Weighting matrix
X Design matrix
T
Transposed design matrix
X
x Discontinuity point
i
y Arithmetic mean of test results of an unspecified laboratory
y Measurement result for the i-th measurement made by laboratory j
ji
y Arithmetic mean of measurement results of laboratory j
j
z z-Score, i.e. standardized deviation of laboratory result from assigned value
z Corrected z-score
U
Function for determining the Hampel estimator
Ψ
2
95 % chi-squared quantile for p − 2 degrees of freedom
χ
p−2;0,95
Distribution function of standard normal distribution
Φ
Significance level
α
Relative reproducibility standard deviation
ν
γ Vector of logarithms of standard deviations
θ ,θ Parameters of log-linear variance function
0 1

Estimate of logarithm of relative reproducibility standard deviation if independent from
θ
0
concentration level
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ISO/TS 20612:2007(E)
5 Requirements relating to proficiency test provider
Proficiency testing by interlaboratory comparisons must lie in the responsibility of specialists who are familiar
not only with the requirements relating to the design, execution and evaluation of interlaboratory tests, but also
with the analytical methods to be tested, and who have demonstrated their specialist knowledge. Against this
background, it is recommended that the test provider regularly organizes interlaboratory tests in the relevant
test field.
The proficiency test provider must maintain an adequately documented quality management system based on
the criteria specified in ISO/IEC 17020 or ISO/IEC 17025, covering all necessary framework conditions,
responsibilities and standard operation procedures.
In addition all measurements within the framework of the provided proficiency test should fulfil the technical
requirements as specified in ISO/IEC 17025.
An advisory group that includes specialists for all the fields involved should be appointed to enable the
relevant interlaboratory test system to be brought into line with the state of the art and proper account to be
taken of the specialist requirements relating to the interlaboratory tests. Keeping a written record of the
group’s decisions is recommended.
6 Participants
Only laboratories that have the requisite staffing and equipment for the tests to be performed shall take part in
an interlaboratory test. Each participating laboratory should appoint a member of staff to be responsible for
maintaining contact with the proficiency test manager and ensuring that the analyses are correctly carried out
in accordance with the proficiency test manager's instructions.
7 Proficiency test design
7.1 Proficiency test plan
All details of the proficiency test design should be laid down in a plan prior to the start of the interlaboratory
test. This includes especially details about:
⎯ involved staff;
⎯ sample matrix;
⎯ determinands to be analysed;
⎯ concentration level of the determinand;
⎯ number of samples;
⎯ sample containers;
⎯ sample preservation;
⎯ distribution of samples;
⎯ communication with participants of the proficiency test (PT);
⎯ homogenization method;
⎯ homogeneity and stability check;
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ISO/TS 20612:2007(E)
⎯ method for stipulating the assigned value;
⎯ schedule;
⎯ evaluation and assessment procedure.
All relevant practices listed in ISO Guide 43-1 shall be fulfilled.
7.2 Sample selection
In selecting the sample material, account shall be taken of the objectives of the interlaboratory test, the target
concentration levels, the required homogeneity and stability of the samples, and the transport and storage
facilities. In general, real or spiked real samples shall be given preference over synthetic ones. Sample matrix
and concentration levels should reflect routine conditions.
7.3 Selection of determinands
The determinands selected in a particular case and their number shall be defined precisely in accordance with
the target group of participants or with the reason for the interlaboratory test. Determinands shall be defined
accurately, i.e. whether a certain form (e.g. soluble) or the total concentration shall be determined.
7.4 Spiking
For the preparation of samples, the proficiency test provider may spike samples with low concentrations. This
can be a useful way of establishing required combinations of concentrations of individual analytes in samples.
However, it does not make sense or may not be possible in all cases, especially if the type of analyte binding
in the original sample is significantly different from that in the spiked solutions and the degree of difficulty in
performing the analytical methods is altered.
7.5 Number of participants
If statistical methods are used to calculate an assigned value from participants data, the number of
participants has an influence on the reliability of the statistically calculated data. In this, it is therefore desirable
to ensure that the number of laboratories participating in the interlaboratory tests is sufficiently large and never
less than twelve if the assigned value is derived from the participants data.
7.6 Number of samples
Testing several samples for the same analyte yields a more reliable picture of the proficiency of a laboratory
and it is therefore desirable for the participants to analyse several samples involving different concentrations
of the individual analytes.
Steps shall be taken to ensure that no single participant receives only samples having a high (or low)
concentration.
7.7 Multiple determinations and sample size
To ensure that the interlaboratory tests are performed under conditions that resemble routine operation as
closely as possible, the participants shall make the same number of multiple determinations as in their routine
work. Attention shall be drawn to any specification of the number of parallel determinations required by
regulations or by the proficiency test provider.
To reduce the possibility that multiple determinations are not in line with routine or go beyond the number
specified in the interlaboratory test, the proficiency test provider should, if practicable, limit the sample size to
that required for the specified test.
Dilution of concentrates by the participants prior to testing should be avoided if possible.
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ISO/TS 20612:2007(E)
8 Execution of proficiency tests
8.1 General
A written record should be kept confirming correct implementation of all the requirements of the proficiency
test plan as well as any necessary deviations from the specified procedure.
8.2 Sample preparation
All the steps to be taken in obtaining the sample material, ranging from the selection and cleaning of the
transport vessels, sampling and transportation to the laboratory to dispensing, labeling and packaging the
subsamples, should be documented in standard operation procedures.
If synthetic samples are prepared or real samples are spiked, the proficiency test provider should provide
evidence of the suitability of the materials/substances used in regard to traceability of the chemical
composition and the stoichiometry.
All the procedures for ensuring correct spiking, e.g. determination of the pipettes precision or of volume
measurements based on mass, should be clearly documented. In addition, contamination and analyte losses
should be determined and taken into account. Responsibility for these steps should be specified before the
interlaboratory test is started.
The variation in the concentrations of the subsamples should not be excessively increased by the preparation
procedure adopted since the reproducibility standard deviation of the test data would otherwise assume
unrealistically high values. This should be borne in mind, in particular in relation to unstable and highly volatile
analytes.
The containers for samples and subsamples should be such as to ensure that contamination resulting from
the material and losses due to adsorption, outgassing and the like are minimized.
8.3 Stability and homogeneity testing
The proficiency test provider should provide evidence of the stability and homogeneity of subsamples and, in
particular, of the substances to be quantified, for every phase of the interlaboratory test. For this purpose,
additional backup samples to be analysed at suitable time intervals during the interlaboratory test by the test
provider for the purpose of checking stability should be prepared when dispensing the subsamples.
8.4 Prevention of collusion between participants
Examples of possible steps to be taken by the proficiency test provider to prevent improper contacts are given
below:
a) requiring the laboratories to submit copies of the raw data printouts from their analytical equipment along
with the analytical results so that the proficiency test manager can use them to perform plausibility tests;
b) each participant receives a subset of the samples prepared (e.g. 3 out of 12);
c) contact accreditation body requiring spot checks to be performed on raw data and other printouts in the
course of auditing in the participants laboratory.
8.5 Analytical methods
Depending on the objective or context of the interlaboratory test, the proficiency test provider may restrict or
specify the analytical methods to be used. If he does not, the person in charge in the participating laboratory
shall use the method normally used by the laboratory for analysing this type of sample.
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ISO/TS 20612:2007(E)
8.6 Specification of the assigned value
There are various ways of specifying the assigned value:
a) by preparing the samples from substances having a precisely known composition (synthetic samples) and
determining the true values from the initial sample mass;
b) by preparing the samples from certified reference materials;
c) by using the results of reference laboratories;
d) by using the robust mean of the participating laboratories.
The proficiency test manager shall be responsible for choosing an optimum method of specifying the assigned
value for a determinand in each individual case, variations and combinations of the abovementioned points
being conceivable or useful. Proceed as described in ISO 13528.
8.7 Sample distribution
The proficiency test provider should organize sample distribution so as to avoid the stability of the subsamples
being adversely affected while they await delivery. This may also mean that the subsamples have to be
collected by the participating laboratories.
Preferably the subsamples should be shipped. Steps should be taken to ensure that the subsamples are
received within a defined time window by all the participants, depending on the stability of the samples. The
dispatch deadline should be such that the subsamples are delivered to all the participants under the specified
conditions and any necessary steps relating to storage and pretreatment can be carried out without delay.
A suitable system should be instituted for checking that deadlines are met.
8.8 Communication with participants
The proficiency test provider should prepare a long-term plan for executing regular interlaboratory tests and
should inform the interested laboratories in due time of when tests are to take place, the number of samples to
be tested and the determinands, including any special features of the sample matrix. The assessment criteria
should also be published before the test is started.
The proficiency test provider should provide the laboratories with the requirements relating to the test
objective (analytes selected), sample pretreatment and, if required, the use of specified analytical methods
(preferably standard methods) or, if applicable, to the possibility of choosing equivalent methods no later than
the date on which the subsamples are delivered.
The results should be reported on standard forms and/or data media that have been supplied. The number of
multiple determinations, the decimal places to be reported, and the units should be specified. The use of these
forms should be obligatory, all the data required shall be entered on them and they shall be authorized by the
person responsible.
The deadline for the submission of the test results shall be specified beforehand and shall be as short as
possible since the measurements are to be performed under largely routine conditions. The laboratories
should also be informed of the time that the proficiency test provider expects will be needed for the evaluation
of the results.
After the test has been evaluated, the proficiency test provider shall inform all the participants of the results of
the assessment of their laboratories and also of the statistical overall evaluation of the interlaboratory test (in
an anonymous form).
The proficiency test provider should arrange meetings for the exchange of information and specialist criticism.
This may promote, for example, the updating of the interlaboratory test system to the state of the art.
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ISO/TS 20612:2007(E)
9 Proficiency test evaluation
1)
9.1 General statistical evaluation procedure
9.1.1 General requirements
The statistical methods described below fulfil the following requirements.
a) The methods enable comparisons to be made over a range of concentrations.
b) The methods are robust in the sense that any outliers have only a limited effect on the overall result.
Steps were taken to ensure that the results are still meaningful even if the proportion of outliers is 1/3, i.e.
the breakdown point is not below 33 %.
c) The methods are fair in regard to the sign of the laboratory error. Adjustments of the analytical results
towards higher or lower values does not result in an increase in the probability of a positive assessment.
d) The methods comply with international requirements, in particular with the joint ISO, IUPAC and AOAC
[1]
protocol and with ISO 13528.
9.1.2 Steps in the evaluation of an interlaboratory test
Evaluation of an interlaboratory test will, as a rule, involve the following four steps.
a) Definition of the standard deviation for proficiency assessment, σˆ
The standard deviation for proficiency assessment, σˆ , serves to calculate the quality limits for the
analytical results. It may be specified as a quality requirement, but it is, as a rule, determined from the
analytical results of the test participants using statistical methods, if it can be assumed that the majority of
the participants competently uses suitable analytical methods. In 9.2.2 the Q-method is described, a
robust statistical method for calculating the reproducibility standard deviation, s .
R
If steps are to be taken to ensure that the true standard deviation calculated in this way is not too wide or
too narrow with regard to the analytical quality requirements, lower and upper limits can be defined for it.
If the calculated true standard deviation is above or below one of these limits, the latter shall be defined
as σˆ .
b) Specification of the assigned value, x
a
As already described in 8.6, the specification of the assigned value depends on the sample preparation
method. In 9.2.3 the Hampel estimator as a robust statistical method for use when the assigned value is
to be determined from the participants’ results is described.

1) Information on suitable software for the procedure and the statistical evaluations described in this International
Standard is obtainable from the Normenausschuss Wasserwesen, DIN, 10772 Berlin, Germany.
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ISO/TS 20612:2007(E)
c) Calculation and optional use of a variance function
In interlaboratory tests for assessing laboratories proficiency, samples of the same type having different
concentrations are often distributed. A single evaluation of these various samples frequently reveals
fluctuations in the variance of the different concentration levels that might result in laboratories being
unfairly assessed. To be able to correct for s
...

SLOVENSKI STANDARD
SIST-TS ISO/TS 20612:2010
01-september-2010
Kakovost vode - Medlaboratorijske primerjave za ugotavljanje usposobljenosti
analiznih kemijskih laboratorijev
Water quality - Interlaboratory comparisons for proficiency testing of analytical chemistry
laboratories
Qualité de l'eau - Comparaisons interlaboratoires pour des essais de compétence de
laboratoires de chimie analytique
Ta slovenski standard je istoveten z: ISO/TS 20612:2007
ICS:
13.060.45 Preiskava vode na splošno Examination of water in
general
SIST-TS ISO/TS 20612:2010 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST-TS ISO/TS 20612:2010

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SIST-TS ISO/TS 20612:2010

TECHNICAL ISO/TS
SPECIFICATION 20612
First edition
2007-12-01

Water quality — Interlaboratory
comparisons for proficiency testing of
analytical chemistry laboratories
Qualité de l'eau — Comparaisons interlaboratoires pour des essais de
compétence de laboratoires de chimie analytique




Reference number
ISO/TS 20612:2007(E)
©
ISO 2007

---------------------- Page: 3 ----------------------

SIST-TS ISO/TS 20612:2010
ISO/TS 20612:2007(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


COPYRIGHT PROTECTED DOCUMENT


©  ISO 2007
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 either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland

ii © ISO 2007 – All rights reserved

---------------------- Page: 4 ----------------------

SIST-TS ISO/TS 20612:2010
ISO/TS 20612:2007(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope .1
2 Normative references .1
3 Terms and definitions .2
4 Symbols .2
5 Requirements relating to proficiency test provider .4
6 Participants .4
7 Proficiency test design .4
8 Execution of proficiency tests.6
9 Proficiency test evaluation .8
10 Presentation of results.15
11 Archiving and managing the results.15
Annex A (informative) Example of Q-method estimation principle.16
Annex B (informative) Determination of repeatability standard deviation, s .18
r
Annex C (informative) Example of evaluation method in Clause 9.19
Annex D (informative) Example of variance function calculation (9.3).21
Bibliography .25

© ISO 2007 – All rights reserved iii

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SIST-TS ISO/TS 20612:2010
ISO/TS 20612:2007(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member 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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. 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.
In other circumstances, particularly when there is an urgent market requirement for such documents, a
technical committee may decide to publish other types of normative document:
In other circumstances, particularly when there is an urgent market requirement for such documents, a
technical committee may decide to publish other types of document:
⎯ an ISO Publicly Available Specification (ISO/PAS) represents an agreement between technical experts in
an ISO working group and is accepted for publication if it is approved by more than 50 % of the members
of the parent committee casting a vote;
⎯ an ISO Technical Specification (ISO/TS) represents an agreement between the members of a technical
committee and is accepted for publication if it is approved by 2/3 of the members of the committee casting
a vote.
An ISO/PAS or ISO/TS is reviewed after three years in order to decide whether it will be confirmed for a
further three years, revised to become an International Standard, or withdrawn. If the ISO/PAS or ISO/TS is
confirmed, it is reviewed again after a further three years, at which time it must either be transformed into an
International Standard or be withdrawn.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TS 20612 was prepared by Technical Committee ISO/TC 147, Water quality, Subcommittee SC 2,
Physical, chemical and biochemical methods.
iv © ISO 2007 – All rights reserved

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SIST-TS ISO/TS 20612:2010
ISO/TS 20612:2007(E)
Introduction
Participation in interlaboratory tests in various test fields offers a testing laboratory an opportunity to obtain an
objective picture of its proficiency. Such tests serve as a confidence-building measure both for the laboratory
itself and for prospective clients.
This Technical Specification is based on the following international recognized documents:
⎯ ISO/IEC Guides 43-1 and 43-2;
⎯ ISO 13528;
⎯ The International Harmonized Protocol for the Proficiency Testing of Analytical Chemistry Laboratories –
(IUPAC, ISO, AOAC);
⎯ ILAC Guide 13;
⎯ ISO/IEC 17025;
⎯ ISO 5725-1 and ISO 5725-2.
As these documents only define a framework for design, execution and evaluation of proficiency testing by
interlaboratory comparisons, this Technical Specification describes in detail an evaluation procedure which is
especially suitable for the sector of water, waste water and sludge analysis, where results of interlaboratory
comparisons play an important role in the admission of laboratories to certain analytical tasks. Therefore, the
fairness of assessment of laboratories must be guaranteed. Assessment should not be dependent on the
provider, the date, or the method of evaluation.

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SIST-TS ISO/TS 20612:2010
TECHNICAL SPECIFICATION ISO/TS 20612:2007(E)

Water quality — Interlaboratory comparisons for proficiency
testing of analytical chemistry laboratories
1 Scope
This Technical Specification specifies the criteria related to proficiency testing by interlaboratory comparisons
in the field of water, waste water and sludge analysis. In particular, it specifies the requirements in respect to
proficiency test providers and to the design, execution and evaluation of laboratory proficiency comparisons.
This document may be used if the determinands in the interlaboratory test may be regarded as capable of
measurement with a certain degree of continuity. This is generally the case for chemical constituents and
physicochemical determinands, but continuity does not always exist in the case of biological and/or
microbiological determinands.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 3534-1, Statistics — Vocabulary and symbols — Part 1: General statistical terms and terms used in
probability
ISO 5725-1, Accuracy (trueness and precision) of measurement methods and results — Part 1: General
principles and definitions
ISO 5725-1:1994/Cor.1:1998, Accuracy (trueness and precision) of measurement methods and results —
Part 1: General principles and definitions — Technical Corrigendum 1
ISO 5725-2, 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
ISO 13528, Statistical methods for use in proficiency testing by interlaboratory comparisons
ISO/IEC Guide 43-1, Proficiency testing by interlaboratory comparisons — Part 1: Development and operation
of proficiency testing schemes
ISO/IEC Guide 43-2, Proficiency testing by interlaboratory comparisons — Part 2: Selection and use of
proficiency testing schemes by laboratory accreditation bodies
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
ISO/IEC 17020, General criteria for the operation of various types of bodies performing inspection
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SIST-TS ISO/TS 20612:2010
ISO/TS 20612:2007(E)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3534-1, ISO 5725-1, ISO 5725-2,
ISO/IEC Guide 43-1, ISO/IEC Guide 43-2 and the following apply.
3.1
breakdown point
smallest percentage of outlier laboratories above which the estimation method may be entirely inapplicable
3.2
efficiency
ratio of the variance of the optimum estimation method for normal distribution to the variance of the estimation
method under consideration, each assuming a normal distribution
NOTE This is expressed as a percentage.
3.3
sample
totality of a homogeneous analysis material with an identical composition or quality (similar to term batch)
3.4
subsample
defined portion of a sample obtained by suitable sample division and identical in terms of composition
4 Symbols
d Absolute difference between log-linear variance function and the logarithm of the reproducibility
i
standard deviation
G (x ) Generalized distribution function of interlaboratory differences with continuity correction (s )
1 i R
G (x ) Generalized distribution function of intralaboratory differences with continuity correction (s )
2 i r
g Quality limit
H (x ) Generalized distribution function of interlaboratory differences (s )
1 i R
H (x ) Generalized distribution function of intralaboratory differences (s )
2 i r
i
Index denoting the serial number of one of p samples
J Number of participants in the case of sample i
i
j Index denoting the serial number of one of J participating laboratories
k ,k Correction factors for calculating z -score
1 2 U
Overall mean
µ
Overall mean of i-th sample
µ
i
x Assigned value
a
n Number of measurements made by laboratory j
j
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SIST-TS ISO/TS 20612:2010
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q Quantile parameter
p
Number of samples (levels)
PG ,PG Testing values for testing variance function
0 1
s Reproducibility standard deviation
R
Reproducibility standard deviation of i-th sample
s
R
i
Reproducibility standard deviation for sample i adjusted using a variance function
sˆ
R
i
s Repeatability standard deviation
r
Standard deviation for proficiency assessment
σˆ
W Weighting matrix
X Design matrix
T
Transposed design matrix
X
x Discontinuity point
i
y Arithmetic mean of test results of an unspecified laboratory
y Measurement result for the i-th measurement made by laboratory j
ji
y Arithmetic mean of measurement results of laboratory j
j
z z-Score, i.e. standardized deviation of laboratory result from assigned value
z Corrected z-score
U
Function for determining the Hampel estimator
Ψ
2
95 % chi-squared quantile for p − 2 degrees of freedom
χ
p−2;0,95
Distribution function of standard normal distribution
Φ
Significance level
α
Relative reproducibility standard deviation
ν
γ Vector of logarithms of standard deviations
θ ,θ Parameters of log-linear variance function
0 1

Estimate of logarithm of relative reproducibility standard deviation if independent from
θ
0
concentration level
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5 Requirements relating to proficiency test provider
Proficiency testing by interlaboratory comparisons must lie in the responsibility of specialists who are familiar
not only with the requirements relating to the design, execution and evaluation of interlaboratory tests, but also
with the analytical methods to be tested, and who have demonstrated their specialist knowledge. Against this
background, it is recommended that the test provider regularly organizes interlaboratory tests in the relevant
test field.
The proficiency test provider must maintain an adequately documented quality management system based on
the criteria specified in ISO/IEC 17020 or ISO/IEC 17025, covering all necessary framework conditions,
responsibilities and standard operation procedures.
In addition all measurements within the framework of the provided proficiency test should fulfil the technical
requirements as specified in ISO/IEC 17025.
An advisory group that includes specialists for all the fields involved should be appointed to enable the
relevant interlaboratory test system to be brought into line with the state of the art and proper account to be
taken of the specialist requirements relating to the interlaboratory tests. Keeping a written record of the
group’s decisions is recommended.
6 Participants
Only laboratories that have the requisite staffing and equipment for the tests to be performed shall take part in
an interlaboratory test. Each participating laboratory should appoint a member of staff to be responsible for
maintaining contact with the proficiency test manager and ensuring that the analyses are correctly carried out
in accordance with the proficiency test manager's instructions.
7 Proficiency test design
7.1 Proficiency test plan
All details of the proficiency test design should be laid down in a plan prior to the start of the interlaboratory
test. This includes especially details about:
⎯ involved staff;
⎯ sample matrix;
⎯ determinands to be analysed;
⎯ concentration level of the determinand;
⎯ number of samples;
⎯ sample containers;
⎯ sample preservation;
⎯ distribution of samples;
⎯ communication with participants of the proficiency test (PT);
⎯ homogenization method;
⎯ homogeneity and stability check;
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⎯ method for stipulating the assigned value;
⎯ schedule;
⎯ evaluation and assessment procedure.
All relevant practices listed in ISO Guide 43-1 shall be fulfilled.
7.2 Sample selection
In selecting the sample material, account shall be taken of the objectives of the interlaboratory test, the target
concentration levels, the required homogeneity and stability of the samples, and the transport and storage
facilities. In general, real or spiked real samples shall be given preference over synthetic ones. Sample matrix
and concentration levels should reflect routine conditions.
7.3 Selection of determinands
The determinands selected in a particular case and their number shall be defined precisely in accordance with
the target group of participants or with the reason for the interlaboratory test. Determinands shall be defined
accurately, i.e. whether a certain form (e.g. soluble) or the total concentration shall be determined.
7.4 Spiking
For the preparation of samples, the proficiency test provider may spike samples with low concentrations. This
can be a useful way of establishing required combinations of concentrations of individual analytes in samples.
However, it does not make sense or may not be possible in all cases, especially if the type of analyte binding
in the original sample is significantly different from that in the spiked solutions and the degree of difficulty in
performing the analytical methods is altered.
7.5 Number of participants
If statistical methods are used to calculate an assigned value from participants data, the number of
participants has an influence on the reliability of the statistically calculated data. In this, it is therefore desirable
to ensure that the number of laboratories participating in the interlaboratory tests is sufficiently large and never
less than twelve if the assigned value is derived from the participants data.
7.6 Number of samples
Testing several samples for the same analyte yields a more reliable picture of the proficiency of a laboratory
and it is therefore desirable for the participants to analyse several samples involving different concentrations
of the individual analytes.
Steps shall be taken to ensure that no single participant receives only samples having a high (or low)
concentration.
7.7 Multiple determinations and sample size
To ensure that the interlaboratory tests are performed under conditions that resemble routine operation as
closely as possible, the participants shall make the same number of multiple determinations as in their routine
work. Attention shall be drawn to any specification of the number of parallel determinations required by
regulations or by the proficiency test provider.
To reduce the possibility that multiple determinations are not in line with routine or go beyond the number
specified in the interlaboratory test, the proficiency test provider should, if practicable, limit the sample size to
that required for the specified test.
Dilution of concentrates by the participants prior to testing should be avoided if possible.
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8 Execution of proficiency tests
8.1 General
A written record should be kept confirming correct implementation of all the requirements of the proficiency
test plan as well as any necessary deviations from the specified procedure.
8.2 Sample preparation
All the steps to be taken in obtaining the sample material, ranging from the selection and cleaning of the
transport vessels, sampling and transportation to the laboratory to dispensing, labeling and packaging the
subsamples, should be documented in standard operation procedures.
If synthetic samples are prepared or real samples are spiked, the proficiency test provider should provide
evidence of the suitability of the materials/substances used in regard to traceability of the chemical
composition and the stoichiometry.
All the procedures for ensuring correct spiking, e.g. determination of the pipettes precision or of volume
measurements based on mass, should be clearly documented. In addition, contamination and analyte losses
should be determined and taken into account. Responsibility for these steps should be specified before the
interlaboratory test is started.
The variation in the concentrations of the subsamples should not be excessively increased by the preparation
procedure adopted since the reproducibility standard deviation of the test data would otherwise assume
unrealistically high values. This should be borne in mind, in particular in relation to unstable and highly volatile
analytes.
The containers for samples and subsamples should be such as to ensure that contamination resulting from
the material and losses due to adsorption, outgassing and the like are minimized.
8.3 Stability and homogeneity testing
The proficiency test provider should provide evidence of the stability and homogeneity of subsamples and, in
particular, of the substances to be quantified, for every phase of the interlaboratory test. For this purpose,
additional backup samples to be analysed at suitable time intervals during the interlaboratory test by the test
provider for the purpose of checking stability should be prepared when dispensing the subsamples.
8.4 Prevention of collusion between participants
Examples of possible steps to be taken by the proficiency test provider to prevent improper contacts are given
below:
a) requiring the laboratories to submit copies of the raw data printouts from their analytical equipment along
with the analytical results so that the proficiency test manager can use them to perform plausibility tests;
b) each participant receives a subset of the samples prepared (e.g. 3 out of 12);
c) contact accreditation body requiring spot checks to be performed on raw data and other printouts in the
course of auditing in the participants laboratory.
8.5 Analytical methods
Depending on the objective or context of the interlaboratory test, the proficiency test provider may restrict or
specify the analytical methods to be used. If he does not, the person in charge in the participating laboratory
shall use the method normally used by the laboratory for analysing this type of sample.
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8.6 Specification of the assigned value
There are various ways of specifying the assigned value:
a) by preparing the samples from substances having a precisely known composition (synthetic samples) and
determining the true values from the initial sample mass;
b) by preparing the samples from certified reference materials;
c) by using the results of reference laboratories;
d) by using the robust mean of the participating laboratories.
The proficiency test manager shall be responsible for choosing an optimum method of specifying the assigned
value for a determinand in each individual case, variations and combinations of the abovementioned points
being conceivable or useful. Proceed as described in ISO 13528.
8.7 Sample distribution
The proficiency test provider should organize sample distribution so as to avoid the stability of the subsamples
being adversely affected while they await delivery. This may also mean that the subsamples have to be
collected by the participating laboratories.
Preferably the subsamples should be shipped. Steps should be taken to ensure that the subsamples are
received within a defined time window by all the participants, depending on the stability of the samples. The
dispatch deadline should be such that the subsamples are delivered to all the participants under the specified
conditions and any necessary steps relating to storage and pretreatment can be carried out without delay.
A suitable system should be instituted for checking that deadlines are met.
8.8 Communication with participants
The proficiency test provider should prepare a long-term plan for executing regular interlaboratory tests and
should inform the interested laboratories in due time of when tests are to take place, the number of samples to
be tested and the determinands, including any special features of the sample matrix. The assessment criteria
should also be published before the test is started.
The proficiency test provider should provide the laboratories with the requirements relating to the test
objective (analytes selected), sample pretreatment and, if required, the use of specified analytical methods
(preferably standard methods) or, if applicable, to the possibility of choosing equivalent methods no later than
the date on which the subsamples are delivered.
The results should be reported on standard forms and/or data media that have been supplied. The number of
multiple determinations, the decimal places to be reported, and the units should be specified. The use of these
forms should be obligatory, all the data required shall be entered on them and they shall be authorized by the
person responsible.
The deadline for the submission of the test results shall be specified beforehand and shall be as short as
possible since the measurements are to be performed under largely routine conditions. The laboratories
should also be informed of the time that the proficiency test provider expects will be needed for the evaluation
of the results.
After the test has been evaluated, the proficiency test provider shall inform all the participants of the results of
the assessment of their laboratories and also of the statistical overall evaluation of the interlaboratory test (in
an anonymous form).
The proficiency test provider should arrange meetings for the exchange of information and specialist criticism.
This may promote, for example, the updating of the interlaboratory test system to the state of the art.
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9 Proficiency test evaluation
1)
9.1 General statistical evaluation procedure
9.1.1 General requirements
The statistical methods described below fulfil the following requirements.
a) The methods enable comparisons to be made over a range of concentrations.
b) The methods are robust in the sense that any outliers have only a limited effect on the overall result.
Steps were taken to ensure that the results are still meaningful even if the proportion of outliers is 1/3, i.e.
the breakdown point is not below 33 %.
c) The methods are fair in regard to the sign of the laboratory error. Adjustments of the analytical results
towards higher or lower values does not result in an increase in the probability of a positive assessment.
d) The methods comply with international requirements, in particular with the joint ISO, IUPAC and AOAC
[1]
protocol and with ISO 13528.
9.1.2 Steps in the evaluation of an interlaboratory test
Evaluation of an interlaboratory test will, as a rule, involve the following four steps.
a) Definition of the standard deviation for proficiency assessment, σˆ
The standard deviation for proficiency assessment, σˆ , serves to calculate the quality limits for the
analytical results. It may be specified as a quality requirement, but it is, as a rule, determined from the
analytical results of the test participants using statistical methods, if it can be assumed that the majority of
the participants competently uses suitable analytical methods. In 9.2.2 the Q-method is described, a
robust statistical method for calculating the reproducibility standard deviation, s .
R
If steps are to be taken to ensure that the true standard deviation calculated in this way is not too wide or
too narrow with regard to the analytical quality requirements, lower and upper limi
...