Microbiology of the food chain - Specific requirements and guidance for proficiency testing by interlaboratory comparison (ISO 22117:2019)

This document specifies requirements and gives guidelines for the organization of proficiency testing (PT) schemes for microbiological examinations of
a) foods and beverages,
b) feeding animals,
c) environmental samples from food and feed production and handling, and
d) primary production stages.
This document is also applicable to the microbiological examination of water where water is either used in food production or is regarded as a food in national legislation.
This document relates to the technical organization and implementation of PT schemes, as well as the statistical treatment of results of microbiological examinations.
This document is designed for use with ISO/IEC 17043 and ISO 13528, and deals only with areas where specific or additional details are necessary for PT schemes dealing with microbiological examinations for the areas specified in the first paragraph.

Mikrobiologie der Lebensmittelkette - Spezielle Anforderungen und Anleitungen an die Eignungsprüfung durch Ringversuche (ISO 22117:2019)

Dieses Dokument legt Anforderungen fest und enthält eine Anleitung für die Organisation von Eignungsprüfungsprogrammen (Eignungsprüfung, en: proficieny testing, abgekürzt mit PT) für die mikrobiologische Untersuchung von:
a)   Lebensmitteln und Getränken;
b)   Futtermitteln;
c)   Umgebungsproben aus der Lebensmittel- und Futtermittelherstellung und -handhabung, und
d)   Primärproduktionsstufen.
Dieses Dokument ist auch für die mikrobiologischen Untersuchungen von Wasser anwendbar, wenn Wasser entweder in der Lebensmittelproduktion verwendet oder in der nationalen Gesetzgebung als Lebensmittel angesehen wird.
Dieses Dokument bezieht sich auf die technische Organisation und die Umsetzung von Eignungsprüfungs-programmen sowie auf die statistische Behandlung von Ergebnissen mikrobiologischer Untersuchungen.
Dieses Dokument ist für die Anwendung in Verbindung mit ISO/IEC 17043 und ISO 13528 vorgesehen und behandelt nur die Gebiete, in denen spezifische oder zusätzliche Einzelheiten für Eignungsprüfungs¬pro-gramme, die sich mit mikrobiologischen Untersuchungen in den im ersten Absatz aufgeführten Bereichen befassen, notwendig sind.

Microbiologie de la chaîne alimentaire - Exigences spécifiques et recommandations relatives aux essais d'aptitude par comparaison interlaboratoires (ISO 22117:2019)

Le présent document spécifie des exigences et fournit des lignes directrices relatives à l'organisation de programmes d'essai d'aptitude pour les examens microbiologiques concernant:
a) les aliments et les boissons;
b) l'alimentation animale;
c) les échantillons environnementaux prélevés dans les secteurs de la production et de la manutention des aliments;
d) les étapes de production primaire.
Le présent document peut également s'appliquer à l'examen microbiologique de l'eau si celle-ci est utilisée dans la production alimentaire ou si la législation nationale la considère comme un aliment.
Le présent document concerne l'organisation technique et la mise en œuvre de programmes d'essai d'aptitude et également le traitement statistique des résultats des examens microbiologiques.
Le présent document est destiné à être utilisé avec l'ISO/IEC 17043 et l'ISO 13528 et ne traite que des domaines où des détails spécifiques ou supplémentaires sont nécessaires pour les programmes d'essai d'aptitude traitant des examens microbiologiques pour les domaines spécifiés dans le premier alinéa.

Mikrobiologija v prehranski verigi - Posebne zahteve in napotki za preskušanje strokovnosti z medlaboratorijsko primerjavo (ISO 22117:2019)

Ta dokument določa zahteve in podaja smernice za urejanje shem preskušanja strokovnosti (PT) za mikrobiološke preiskave
a) hrane in pijače,
b) krmljenja živali,
c) okoljskih vzorcev hrane ter proizvodnje in ravnanja s hrano; in
d) primarnih stopenj proizvodnje.
Ta dokument se uporablja tudi za mikrobiološko preiskovanje vode, ki se uporablja v proizvodnji hrane oziroma se v nacionalni zakonodaji obravnava kot živilo.
Ta dokument se nanaša na tehnično organizacijo in izvajanje shem preskušanja strokovnosti ter statistično obdelavo rezultatov mikrobioloških preiskav.
Ta dokument je zasnovan za uporabo s standardoma ISO/IEC 17043 in ISO 13528 ter obravnava samo področja, kjer so potrebne posebne ali dodatne podrobnosti za sheme preskušanja strokovnosti, ki zajemajo mikrobiološke preiskave za področja, opredeljena v prvem odstavku.

General Information

Status
Published
Publication Date
19-Mar-2019
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Due Date
20-Mar-2019
Completion Date
20-Mar-2019

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SLOVENSKI STANDARD
SIST EN ISO 22117:2019
01-maj-2019
1DGRPHãþD
SIST-TS CEN ISO/TS 22117:2011
Mikrobiologija v prehranski verigi - Posebne zahteve in napotki za preskušanje
strokovnosti z medlaboratorijsko primerjavo (ISO 22117:2019)

Microbiology of the food chain - Specific requirements and guidance for proficiency

testing by interlaboratory comparison (ISO 22117:2019)

Mikrobiologie der Lebensmittelkette - Spezielle Anforderungen und Anleitungen an die

Eignungsprüfung durch Ringversuche (ISO 22117:2019)

Microbiologie de la chaîne alimentaire - Exigences spécifiques et recommandations

relatives aux essais d'aptitude par comparaison interlaboratoires (ISO 22117:2019)

Ta slovenski standard je istoveten z: EN ISO 22117:2019
ICS:
07.100.30 Mikrobiologija živil Food microbiology
SIST EN ISO 22117:2019 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 22117:2019
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SIST EN ISO 22117:2019
EN ISO 22117
EUROPEAN STANDARD
NORME EUROPÉENNE
March 2019
EUROPÄISCHE NORM
ICS 07.100.30 Supersedes CEN ISO/TS 22117:2010
English Version
Microbiology of the food chain - Specific requirements and
guidance for proficiency testing by interlaboratory
comparison (ISO 22117:2019)

Microbiologie de la chaîne alimentaire - Exigences Mikrobiologie der Lebensmittelkette - Spezielle

spécifiques et recommandations relatives aux essais Anforderungen und Anleitungen an die

d'aptitude par comparaison interlaboratoires (ISO Eignungsprüfung durch Ringversuche (ISO

22117:2019) 22117:2019)
This European Standard was approved by CEN on 19 February 2019.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this

European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references

concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN

member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by

translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management

Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels

© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 22117:2019 E

worldwide for CEN national Members.
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SIST EN ISO 22117:2019
EN ISO 22117:2019 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 22117:2019
EN ISO 22117:2019 (E)
European foreword

This document (EN ISO 22117:2019) has been prepared by Technical Committee ISO/TC 34 "Food

products" in collaboration with Technical Committee CEN/TC 275 “Food analysis - Horizontal methods”

the secretariat of which is held by DIN.

This European Standard shall be given the status of a national standard, either by publication of an

identical text or by endorsement, at the latest by September 2019, and conflicting national standards

shall be withdrawn at the latest by September 2019.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CEN shall not be held responsible for identifying any or all such patent rights.

This document supersedes CEN ISO/TS 22117:2010.

According to the CEN-CENELEC Internal Regulations, the national standards organizations of the

following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,

Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,

France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,

Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and the United Kingdom.
Endorsement notice

The text of ISO 22117:2019 has been approved by CEN as EN ISO 22117:2019 without any modification.

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SIST EN ISO 22117:2019
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SIST EN ISO 22117:2019
INTERNATIONAL ISO
STANDARD 22117
First edition
2019-02
Microbiology of the food chain —
Specific requirements and
guidance for proficiency testing by
interlaboratory comparison
Microbiologie de la chaîne alimentaire — Exigences spécifiques et
recommandations relatives aux essais d'aptitude par comparaison
interlaboratoires
Reference number
ISO 22117:2019(E)
ISO 2019
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SIST EN ISO 22117:2019
ISO 22117:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2019

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
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SIST EN ISO 22117:2019
ISO 22117:2019(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

Introduction ................................................................................................................................................................................................................................vi

1 Scope ................................................................................................................................................................................................................................. 1

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

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Scheme design and purpose ..................................................................................................................................................................... 2

4.1 General ........................................................................................................................................................................................................... 2

4.2 Scheme objectives ................................................................................................................................................................................ 2

4.3 Laboratory requirements for schemes ............................................................................................................................... 3

4.4 Choice of test matrices ..................................................................................................................................................................... 3

4.5 Information on test methods used by the PT provider ........................................................................................ 3

4.6 Statistical design .................................................................................................................................................................................... 3

5 Technical requirements and guidance for sample design and content .......................................................4

5.1 Sources, characterization and traceability of organisms .................................................................................... 4

5.2 Target organisms level ..................................................................................................................................................................... 4

5.3 Non-target organisms and interferences ......................................................................................................................... 5

5.4 Matrix selection and effects ......................................................................................................................................................... 5

6 Sample verification by the provider ................................................................................................................................................. 6

6.1 General ........................................................................................................................................................................................................... 6

6.2 Sample homogeneity testing — General considerations .................................................................................... 6

6.3 Homogeneity testing for quantitative (enumeration) samples .................................................................... 6

6.4 Homogeneity testing for qualitative methods ............................................................................................................. 7

6.5 Stability testing by the provider .............................................................................................................................................. 8

6.5.1 General...................................................................................................................................................................................... 8

6.5.2 Stability during storage conditions .................. ................................................................................................ 8

6.5.3 Stability during transport conditions ............................................................................................................ 8

7 Sample handling ................................................................................................................................................................................................... 9

7.1 General ........................................................................................................................................................................................................... 9

7.2 Instructions to participants ......................................................................................................................................................... 9

8 Performance evaluations ............................................................................................................................................................................. 9

8.1 General ........................................................................................................................................................................................................... 9

8.2 Preliminary considerations .......................................................................................................................................................... 9

8.3 Assessment of quantitati ve methods ................................................................................................................................10

8.3.1 General...................................................................................................................................................................................10

8.3.2 Distribution of data ....................................................................................................................................................11

8.3.3 Determining the assigned value ......................................................................................................................12

8.3.4 Uncertainty of the assigned value ..................................................................................................................12

8.3.5 Methods of assessing performance ..............................................................................................................12

8.3.6 Using z-scores ..................................................................................................................................................................12

8.3.7 Other methods of performance evaluation ............................................................................................14

8.3.8 Long-term performance assessment ...........................................................................................................16

8.4 Assessment of qualitati ve methods....................................................................................................................................17

8.4.1 General...................................................................................................................................................................................17

8.4.2 Performance of individual laboratories ....................................................................................................17

8.4.3 Scheme comparisons of laboratory performance ............................................................................19

Annex A (informative) Example of details to be included in a PT scheme plan ....................................................21

Annex B (informative) Preparation of fungal spore suspensions .......................................................................................23

Annex C (informative) Methods of testing for variation between portions of test materials .................24

Annex D (informative) Example of a safety data sheet ...................................................................................................................28

© ISO 2019 – All rights reserved iii
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SIST EN ISO 22117:2019
ISO 22117:2019(E)

Annex E (informative) A practical method to assess long-term performance of participants

in PT schemes using enumeration methods .........................................................................................................................30

Bibliography .............................................................................................................................................................................................................................32

iv © ISO 2019 – All rights reserved
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SIST EN ISO 22117:2019
ISO 22117:2019(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.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).

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. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso

.org/iso/foreword .html.

This document was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 9,

Microbiology.

This first edition cancels and replaces ISO/TS 22117:2010, which has been technically revised. The

following changes have been made:
— updates have been made to align the document with ISO 13528:2015.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/members .html.
© ISO 2019 – All rights reserved v
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SIST EN ISO 22117:2019
ISO 22117:2019(E)
Introduction

General requirements for organization of proficiency testing (PT) schemes of all types are given

through ISO/CASCO (Committee on Conformity Assessment) in ISO/IEC 17043. Additionally, general

guidance is available from the International Union of Pure and Applied Chemistry (IUPAC), see

Reference [12]. However, these recommendations may not be directly applicable to all cases and should

be interpreted specifically for different laboratory sectors where PT schemes are organized. For this

reason, a document is needed to establish the criteria for a provider (and associated collaborators) of

PT schemes for microbiological examinations to meet and be recognized as competent. This applies

particularly to the specific technical requirements necessary to deal with microorganisms, such as

sample homogeneity and stability, as well as with the interpretation of detection tests which is not

covered by an existing document.

PT schemes for microbiology laboratories are mainly used to evaluate performance, particularly

trueness (bias) and in some cases precision, of food microbiological examinations in specific

laboratories.
Additionally, data from such PT schemes can be used:

a) to provide information to the organizations responsible for laboratory acceptance within an official

control framework and to allow continuous monitoring;
b) to aid laboratory accreditation in a general framework of quality management;

c) to inform those responsible for quality in the participating laboratories as part of the educative

elements of external quality assessment of trueness (bias).
Information from PT schemes may also be used for:

— identification of the possible sources of errors, particularly the bias component of uncertainty, to

improve performance;

— estimation of uncertainty of test results, in conjunction with routine results, for quantitative

(enumeration) methods (see ISO/TS 19036) and levels of detection for qualitative (detection)

methods;

— demonstration of staff competence to perform a specific microbiological examination;

— evaluation or validation of a given method by the study of trueness, precision and robustness;

— identification of variability in test results between individual laboratories;

— assignment of a “target” value for a microorganism in a material in order to establish a reference

material (see ISO 17034).
However, these aspects are not specifically covered in this document.

PT schemes are therefore designed to meet certain criteria and the testing programme (frequency,

number of samples, number of repeats, etc.) to meet the requirements of the type of method used and

commodity tested, to achieve the level of control required by all parties.
vi © ISO 2019 – All rights reserved
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SIST EN ISO 22117:2019
INTERNATIONAL STANDARD ISO 22117:2019(E)
Microbiology of the food chain — Specific requirements
and guidance for proficiency testing by interlaboratory
comparison
1 Scope

This document specifies requirements and gives guidelines for the organization of proficiency testing

(PT) schemes for microbiological examinations of
a) foods and beverages,
b) feeding animals,
c) environmental samples from food and feed production and handling, and
d) primary production stages.

This document is also applicable to the microbiological examination of water where water is either used

in food production or is regarded as a food in national legislation.

This document relates to the technical organization and implementation of PT schemes, as well as the

statistical treatment of results of microbiological examinations.

This document is designed for use with ISO/IEC 17043 and ISO 13528, and deals only with areas where

specific or additional details are necessary for PT schemes dealing with microbiological examinations

for the areas specified in the first paragraph.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements 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 3534-2, Statistics — Vocabulary and symbols — Part 2: Applied statistics

ISO 5725-1, Accuracy (trueness and precision) of measurement methods and results — Part 1: General

principles and definitions

ISO 13528:2015, Statistical methods for use in proficiency testing by interlaboratory comparison

ISO/IEC 17043:2010, Conformity assessment — General requirements for proficiency testing

3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 3534-1, ISO 3534-2, ISO 5725-1,

ISO 13528, ISO/IEC 17043 and the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
© ISO 2019 – All rights reserved 1
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SIST EN ISO 22117:2019
ISO 22117:2019(E)

NOTE 1 Some terms used in the text have different meanings in microbiology and statistics, e.g. homogeneity,

heterogeneity, test, sample, distribution. The context clarifies whether the terms refer to microbiological test

samples or data sets used for statistical analysis.

NOTE 2 Some providers of proficiency testing use the term “external quality assessment” (EQA) to indicate

schemes with broader application to all areas of operation of a laboratory and a particular educational remit. The

requirements of this document cover those EQA activities that meet the definition of proficiency testing.

3.1
target organism
microorganism that is the designated analyte for a proficiency testing sample
3.2
background flora

microorganisms included in a proficiency testing sample that are naturally present or can be introduced

to compete with or mimic the target microorganism
3.3
matrix
all the components of the sample

[SOURCE: ISO 16140-1:2016, 2.38, modified — In the term, “(product)” has been removed.]

3.4
reference strain

microorganism obtained directly from an official culture collection or reference laboratory and defined

to at least the genus and species level, catalogued and described according to its characteristics and

preferably originating from food, food production areas, primary production stages, animals or water,

as applicable

[SOURCE: ISO 11133:2014, 3.4.2, modified — In the term, “an official culture collection or reference

laboratory” has replaced “a reference culture collection, i.e. a culture collection, which is a member of

the World Federation of Culture Collections (WFCC) or the European Culture Collections’ Organisation

(ECCO)”, and “food production areas, primary production stages, animals" has replaced “animal feed,

the food or feed production environment”.]
3.5
recovery percentage

proportion of the assigned value of the target organism (3.1) recovered by the participant

Note 1 to entry: The recovery percentage is calculated by multiplying by 100 the number of recovered colony-

forming units (cfu) per volume or per mass and dividing by the assigned value.

Note 2 to entry: The recovery percentage can be significantly below 100 % when competitive microflora and

matrix effects are present in a proficiency testing sample.
4 Scheme design and purpose
4.1 General

General requirements for designing PT schemes are given in ISO/IEC 17043. This clause discusses

areas requiring special consideration for microbiological PT schemes in the context of these general

principles.
4.2 Scheme objectives

The primary objective of any PT scheme is to provide information to enable laboratories to have

confidence in the reliability of their results.
2 © ISO 2019 – All rights reserved
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SIST EN ISO 22117:2019
ISO 22117:2019(E)

The detailed requirements for a documented plan of a PT scheme are covered in ISO/IEC 17043:2010,

4.4.1.3, and the plan should also include reference to any relevant legislation. An example of a plan for a

typical microbiology food examination scheme is given in Annex A.

The studies required to establish a new PT scheme are extensive and shall be clearly defined in

the scheme objectives. These should include, as a minimum, the requirements listed in Clause 5.

Requirements for checking individual rounds of testing, including homogeneity and stability testing,

should also be established in the scheme design and be appropriate for the scheme objectives.

4.3 Laboratory requirements for schemes

General requirements for appropriate laboratory facilities to handle all aspects of PT schemes are given

in ISO/IEC 17043:2010, 4.3.1, and safety requirements are covered in ISO/IEC 17043:2010, 4.6.2.4.

For microbiology schemes, providers shall have a documented policy to bring hazards to the attention

of participants and ensure that relevant safety advice is given (see Clause 7). For example, food

microbiology laboratories shall have facilities for dealing with microorganisms of biosafety levels 1

and 2, as appropriate (see ISO 7218).
4.4 Choice of test matrices

General requirements to document test matrices in the scheme plan are given in ISO/IEC 17043:2010,

4.4.1.3, and choice of the matrices to reflect routine sample types in ISO/IEC 17043:2010, 4.4.2.3.

The reasons for the choice of matrix type should be stated (e.g. to provide levels of sample stability and

homogeneity that are fit for the intended purpose of the scheme).

The description of the test items shall specify the sample matrix (natural or simulated); whether

artificially or naturally contaminated; the source and country of origin to comply with international

transport regulations; and any method of preservation used (e.g. freeze-dried, air-dried).

4.5 Information on test methods used by the PT provider

The general requirements for methods to be used by the PT provider are given in ISO/IEC 17043:2010,

4.4.1.3.

If the scheme is targeted at one or more tests specified in or required by legislation, the routine quality

control tests on the scheme samples (e.g. homogeneity and stability) shall be undertaken in accordance

with the methods stipulated in that legislation and this shall be stated (ISO/IEC 17043:2010, 4.5.1).

Participants shall be encouraged to use their routine methods but, where they are undertaking tests

in accordance with legislation, some degree of guidance shall be given, e.g. reference to ISO methods,

legislative texts, or peer-reviewed publications (ISO/IEC 17043:2010, 4.5.1).
4.6 Statistical design

General requirements for statistical design are given in ISO/IEC 17043:2010, 4.4.4.

An outline of the statistical design for PT schemes for microbiology shall indicate that the statistical

tests to be used are influenced by the level of homogeneity of the test material which, in turn, is

influenced by the random variation in distribution of the microorganisms.

Except for low numbers, a log-normal distribution is usually expected in quantitative testing data and

suitable statistical analysis methods shall be used for such data [ISO/IEC 17043:2010, B.3.1.4 d)]. Where

low numbers are required in quan
...

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