Water quality - Requirements for the comparison of the relative recovery of microorganisms by two quantitative methods (ISO 17994:2014)

EN ISO 17994 specifies an evaluation procedure for comparing two methods intended for the quantification of the same target group or species of microorganisms. This International Standard provides the mathematical basis for the evaluation of the average relative performance of two quantitative methods against chosen criteria for the comparison. It does not provide data that would allow an assessment of the precision of the methods being compared. Precision of methods should be assessed as part of their validation. This International Standard does not provide methods for the verification of method performance in a single laboratory.

Wasserbeschaffenheit - Anforderungen für den Vergleich der relativen Wiederfindung von Mikroorganismen durch zwei quantitative Verfahren (ISO 17994:2014)

Diese Internationale Norm legt ein Bewertungsverfahren fest, um zwei Verfahren mit etablierten Leistungsmerkmalen nach ISO/TR 13843 zu vergleichen, die für den Nachweis oder die Quantifizierung derselben Zielgruppe oder Spezies von Mikroorganismen vorgesehen sind.
Diese Internationale Norm liefert die mathematische Grundlage für die Bewertung der durchschnittlichen relativen Leistungsfähigkeit von zwei Verfahren unter Heranziehung festgelegter Gleichwertigkeitskriterien. Sie liefert keine Daten, die eine Bewertung der Präzision der verglichenen Verfahren erlaubt. Es ist zweckmäßig, dass die Genauigkeit der Verfahren im Rahmen ihrer Leistungscharakterisierung beurteilt.
Diese Internationale Norm enthält keine Methoden, um die Leistungsfähigkeit eines Verfahrens in einem einzelnen Labor zu verifizieren.

Qualité de l'eau - Exigences pour la comparaison du rendement relatif des microorganismes par deux méthodes quantitatives (ISO 17994:2014)

L'ISO 17994:2014 spécifie une procédure d'évaluation qui permet de comparer deux méthodes avec des caractéristiques de performance établies conformément à l'ISO/TR 13843 et visant à quantifier le même groupe cible ou la même espèce de micro-organismes.
Elle fournit les bases mathématiques pour évaluer les performances relatives moyennes des deux méthodes quantitatives par rapport à des critères de comparaison déterminés. Elle ne fournit aucune donnée permettant d'évaluer la fidélité des méthodes comparées. Il convient que la fidélité des méthodes soit évaluée lors de la détermination de leurs caractéristiques de performance.
L'ISO 17994:2014 ne propose aucune méthode permettant de vérifier les caractéristiques de performance des méthodes au sein d'un seul laboratoire.

Kakovost vode - Zahteve za primerjavo relativne ponovitve mikroorganizmov z dvema kvantitativnima metodama (ISO 17994:2014)

Standard EN ISO 17994 določa postopek ocenjevanja za primerjavo dveh metod kvantifikacije iste ciljne skupine ali vrste mikroorganizmov. Ta mednarodni standard zagotavlja matematično osnovo za ocenjevanje povprečne relativne učinkovitosti dveh kvantitativnih metod glede na izbrane kriterije primerjave. Ne zagotavlja podatkov, ki bi omogočili ocenjevanje natančnosti primerjanih metod. Natančnost metod naj bi se ocenjevala kot del njihove validacije. Ta mednarodni standard ne zagotavlja metod za preverjanje učinkovitosti metod v enem laboratoriju.

General Information

Status
Published
Public Enquiry End Date
31-Dec-2012
Publication Date
08-Jun-2014
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
14-Mar-2014
Due Date
19-May-2014
Completion Date
09-Jun-2014

RELATIONS

Buy Standard

Standard
SIST EN ISO 17994:2014
English language
30 pages
sale 10% off
Preview
sale 10% off
Preview

e-Library read for
1 day

Standards Content (sample)

SLOVENSKI STANDARD
SIST EN ISO 17994:2014
01-september-2014
1DGRPHãþD
SIST EN ISO 17994:2004
Kakovost vode - Zahteve za primerjavo relativne ponovitve mikroorganizmov z
dvema kvantitativnima metodama (ISO 17994:2014)
Water quality - Requirements for the comparison of the relative recovery of
microorganisms by two quantitative methods (ISO 17994:2014)

Wasserbeschaffenheit - Anforderungen für den Vergleich der relativen Wiederfindung

von Mikroorganismen durch zwei quantitative Verfahren (ISO 17994:2014)
Qualité de l'eau - Exigences pour la comparaison du rendement relatif des
microorganismes par deux méthodes quantitatives (ISO 17994:2014)
Ta slovenski standard je istoveten z: EN ISO 17994:2014
ICS:
07.100.20 Mikrobiologija vode Microbiology of water
13.060.70 Preiskava bioloških lastnosti Examination of biological
vode properties of water
SIST EN ISO 17994:2014 en

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

---------------------- Page: 1 ----------------------
SIST EN ISO 17994:2014
---------------------- Page: 2 ----------------------
SIST EN ISO 17994:2014
EUROPEAN STANDARD
EN ISO 17994
NORME EUROPÉENNE
EUROPÄISCHE NORM
February 2014
ICS 07.100.20; 13.060.70 Supersedes EN ISO 17994:2004
English Version
Water quality - Requirements for the comparison of the relative
recovery of microorganisms by two quantitative methods (ISO
17994:2014)

Qualité de l'eau - Exigences pour la comparaison du Wasserbeschaffenheit - Anforderungen für den Vergleich

rendement relatif des microorganismes par deux méthodes der relativen Wiederfindung von Mikroorganismen durch

quantitatives (ISO 17994:2014) zwei quantitative Verfahren (ISO 17994:2014)
This European Standard was approved by CEN on 8 February 2014.

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, 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: Avenue Marnix 17, B-1000 Brussels

© 2014 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 17994:2014 E

worldwide for CEN national Members.
---------------------- Page: 3 ----------------------
SIST EN ISO 17994:2014
EN ISO 17994:2014 (E)
Contents Page

Foreword ..............................................................................................................................................................3

---------------------- Page: 4 ----------------------
SIST EN ISO 17994:2014
EN ISO 17994:2014 (E)
Foreword

This document (EN ISO 17994:2014) has been prepared by Technical Committee ISO/TC 147 "Water quality"

in collaboration with Technical Committee CEN/TC 230 “Water analysis” 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 August 2014, and conflicting national standards shall be withdrawn at

the latest by August 2014.

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

rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.

This document supersedes EN ISO 17994:2004.

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, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

Endorsement notice

The text of ISO 17994:2014 has been approved by CEN as EN ISO 17994:2014 without any modification.

---------------------- Page: 5 ----------------------
SIST EN ISO 17994:2014
---------------------- Page: 6 ----------------------
SIST EN ISO 17994:2014
INTERNATIONAL ISO
STANDARD 17994
Second edition
2014-02-15
Water quality — Requirements for the
comparison of the relative recovery of
microorganisms by two quantitative
methods
Qualité de l’eau — Exigences pour la comparaison du rendement
relatif des microorganismes par deux méthodes quantitatives
Reference number
ISO 17994:2014(E)
ISO 2014
---------------------- Page: 7 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2014

All rights reserved. Unless otherwise specified, 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
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 2014 – All rights reserved
---------------------- Page: 8 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

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

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

3 Terms, definitions and symbols ............................................................................................................................................................ 1

3.1 Terms and definitions ....................................................................................................................................................................... 1

3.2 Symbols and abbreviated terms............................................................................................................................................... 3

4 Principle ........................................................................................................................................................................................................................ 3

5 Basic requirements for a comparison study ............................................................................................................................ 4

5.1 General ........................................................................................................................................................................................................... 4

5.2 Description of methods ................................................................................................................................................................... 4

5.3 Types of samples ................................................................................................................................................................................... 4

5.4 Number of samples and participating laboratories ................................................................................................ 5

5.5 Counting and confirming................................................................................................................................................................ 7

6 Calculations................................................................................................................................................................................................................ 8

6.1 Preliminary editing of the raw data ...................................................................................................................................... 8

6.2 Basic relative differences ............................................................................................................................................................... 8

6.3 Half-width of the confidence interval .................................................................................................................................. 9

7 Evaluation .................................................................................................................................................................................................................... 9

7.1 Preliminary evaluations .................................................................................................................................................................. 9

7.2 Two-sided evaluation .....................................................................................................................................................................10

7.3 One-sided evaluation ......................................................................................................................................................................12

8 Test report ................................................................................................................................................................................................................13

Annex A (informative) Flowchart ..........................................................................................................................................................................14

Annex B (informative) Comparison studies ...............................................................................................................................................15

Annex C (informative) Derivation of equation for calculation of the number of samples .........................17

Annex D (informative) Example of a two-sided evaluation

........................................................................................................19

Bibliography .............................................................................................................................................................................................................................22

© ISO 2014 – All rights reserved iii
---------------------- Page: 9 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(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, 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, 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 on the meaning of ISO specific terms and expressions related to conformity

assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers

to Trade (TBT) see the following URL: Foreword - Supplementary information

The committee responsible for this document is ISO/TC 147, Water quality, Subcommittee SC 4,

Microbiological methods.

This second edition cancels and replaces the first edition (ISO 17994:2004), which has been technically

revised.
iv © ISO 2014 – All rights reserved
---------------------- Page: 10 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(E)
Introduction

This International Standard specifies criteria and procedures for comparing the average quantitative

results obtained by two microbiological analytical methods, one of which may, but need not, be a

standard or reference method.

The methods considered are based on counts of colonies or of positive and negative liquid enrichment

tubes (MPN).
© ISO 2014 – All rights reserved v
---------------------- Page: 11 ----------------------
SIST EN ISO 17994:2014
---------------------- Page: 12 ----------------------
SIST EN ISO 17994:2014
INTERNATIONAL STANDARD ISO 17994:2014(E)
Water quality — Requirements for the comparison of the
relative recovery of microorganisms by two quantitative
methods

WARNING — Persons using this document should be familiar with normal laboratory practice.

This document does not purport to address all of the safety problems, if any, associated with its

use. It is the responsibility of the user to establish appropriate safety and health practices and to

ensure compliance with any national regulatory conditions.

IMPORTANT — It is absolutely essential that tests conducted in accordance with this document

be carried out by suitably qualified staff.
1 Scope

This International Standard specifies an evaluation procedure for comparing two methods with

established performance characteristics according to ISO/TR 13843 and intended for the quantification

of the same target group or species of microorganisms.

This International Standard provides the mathematical basis for the evaluation of the average relative

performance of two quantitative methods against chosen criteria for the comparison. It does not provide

data for assessment of the precision of the methods being compared. It is appropriate that the precision

of methods is assessed as part of their performance characterization.

This International Standard does not provide methods for the verification of method performance

characterization in a single laboratory.
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

ISO 8199, Water quality — General guidance on the enumeration of micro-organisms by culture

ISO/TR 13843, Water quality — Guidance on validation of microbiological methods
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1.1 General terms
3.1.1.1
comparison trial

interlaboratory method comparison that involves laboratories which perform paired measurements on

several of their own samples with two different methods
© ISO 2014 – All rights reserved 1
---------------------- Page: 13 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(E)
3.1.1.2
not-different method

method considered quantitatively not different to another method when the mean difference between

their confirmed counts and stipulated difference lie between predetermined stipulated limits, taking

into account all sources of variation

Note 1 to entry: This difference can be assessed by the average relative difference of their confirmed counts.

3.1.1.3
predetermined stipulated limit

permitted average difference (based on a “confidence interval” designated −2L to +2L) between results

obtained by each method, based on professional practices or regulatory requirements

Note 1 to entry: Reference [1] suggests that, in international and interlaboratory method performance tests, a

limit of 2L = 10 % for setting the “confidence interval” be the predetermined stipulated limit for drinking water,

and this has been widely used. However, for environmental waters, such as bathing waters, Reference [2] proposes

a predetermined stipulated limit of 2L = 20 %.
3.1.1.4
reference method

method of analysis internationally recognized by experts or by agreement between the parties

Note 1 to entry: As a rule, the reference method is a standard or a commonly used method.

3.1.1.5
standard measurement uncertainty
measurement uncertainty expressed as a standard deviation
(3)
[SOURCE: ISO/IEC Guide 99:2007 , 2.30]
3.1.1.6
trial method
any method which is to be tested for comparison with a reference method
3.1.2 Specific terms
3.1.2.1
count
observed number of objects
EXAMPLE Colonies or cells of microorganisms, plaques of bacteriophages

Note 1 to entry: In this International Standard, the result of an MPN estimation is also considered to be a count.

3.1.2.2
presumptive count

colony count or most probable number (MPN) estimate based on the number of colonies or fermentation

tubes that have an outward appearance that is interpreted as typical of a target organism

3.1.2.3
confirmed count
presumptive count multiplied by the confirmation coefficient
3.1.2.4
relative difference

difference between two results, a and b, measured on a relative (natural logarithmic) scale

2 © ISO 2014 – All rights reserved
---------------------- Page: 14 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(E)
3.2 Symbols and abbreviated terms
A (symbol for the idea of) trial method
a test result by method A
a test result (confirmed count) of method A in sample i
B (symbol for the idea of) reference method
b test result by method B
b test result (confirmed count) of method B in sample i
i subscript indicating a series
k coverage factor used for calculating the confidence interval

L smallest microbiologically significant mean relative difference between the results by

methods A and B
MPN most probable number quantitative method
n number of samples

s experimental standard deviation of the relative difference (standard uncertainty)

s experimental variance

s standard deviation of the relative difference (standard uncertainty) of the mean

W half-width of the confidence interval
x relative difference
x value of the relative difference between a and b in sample i
i i i
arithmetic mean of x (i = 1,2 … n)

x value of the relative difference at the lower confidence limit, derived by subtracting the

value of the half-width of the confidence interval from the mean

x value of the relative difference at the upper confidence limit, derived by adding the value of

the half-width of the confidence interval to the mean
X experimental Poisson index of dispersion

y conditional variable used in computing the number of samples for comparison testing

4 Principle

This International Standard is based on the principle of the paired t-test (see Annex C).

The basic data are pairs of confirmed counts (a , b ) obtained from the examination of two equal portions

i i

taken from the same vessel of a carefully mixed test sample, one determination (count) per method. The

complete design consists of a large number of similar determinations.

In this International Standard, two methods are considered quantitatively “not different” if the mean

relative difference of the paired confirmed counts does not differ significantly from zero and the

© ISO 2014 – All rights reserved 3
---------------------- Page: 15 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(E)

confidence interval does not extend beyond the level of the predetermined stipulated limit. The decision

rules based on the above principle are given in 7.2 and 7.3 and a flow chart is given in Annex A.

5 Basic requirements for a comparison study
5.1 General

Both methods shall have data on detailed performance characteristics, derived in accordance with the

guidance outlined in ISO/TR 13843.

The most important basic requirement of comparison studies is a wide range of samples. Participation

by a number of laboratories is preferable, allowing the expansion of the sample range over large

geographical areas. Also the credibility of a general conclusion is commonly believed to require the

participation of several laboratories. However, the inclusion of a wide range of sample types by a single

laboratory will also be valid. The result of the comparison is generally valid only within the range of

sample types studied. Advice on the conduct of comparison studies is given in Annex B.

It is essential that all laboratories taking part in a collaborative study have recognized quality assurance

systems in use and apply approved basic techniques of cultivation.
5.2 Description of methods

It is important to note that the principles of operation of the two methods being compared should be

well understood and that the significance of any differences in the methods on the outcome of the

comparative assessment should be recognized. This is particularly important if the confirmed results

from each method are based on different principles. Any differences should be detailed in the test report

(see Clause 8).

Performance characteristics data shall be derived in accordance with ISO/TR 13843. Such data for the

methods shall be compared in order to assess potential differences in performance.

EXAMPLE Methods for the enumeration of coliform bacteria based on possession of the enzyme β-galactosidase

have been shown to produce higher counts than those based on the fermentation of lactose due to the detection

of a greater range of coliform bacteria.
5.3 Types of samples

The requirements for method comparisons differ somewhat from the daily routine situation. It is useful

and often necessary to preselect or prepare special samples. Samples for method comparisons shall

contain enough target organisms that the likelihood of scoring a zero count is small.

Samples for method comparisons shall represent types that are included in the scope of both methods.

Natural samples are ideal. Samples to be tested shall represent those water source types relevant to the

geographical and environmental area where the method is applied. The water types to be tested shall

be included to the scope of the methods under evaluation. Appropriate samples may also be prepared

by dilution, spiking, or mixing of different kinds of water to achieve the desired population in a suitable

density. Spiking with pure cultures shall be considered the last resort.

To avoid the inhibition of the target organisms by other organisms, ensure that the concentration of

total bacteria in a sample is not too high. Consult ISO 8199 to ascertain the ranges of the colony counts

for different cultivation methods.

It can be appropriate to influence the microbial population of existing samples to simulate situations

encountered in routine laboratory practice. Such modifications could be the applications of disinfectants

(e.g. chlorine, ozone or UV, Reference [1]), different ranges of temperature or the influence of daylight,

in order to simulate different environmental situations from where the samples for laboratories can

originate.
4 © ISO 2014 – All rights reserved
---------------------- Page: 16 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(E)
5.4 Number of samples and participating laboratories
5.4.1 Number of laboratories

The number of laboratories participating in comparison trials shall be sufficient to obtain a representative

result for the relative recovery of the two methods being tested. Factors that shall be considered when

deciding on the number of participating laboratories in a comparison trial include:

a) whether the alternative method is being assessed as a replacement for the reference method;

b) whether the comparison trials are for statutory or verification purposes;

c) the need to cover the range of geographical areas and water types for which the alternative method

may be used;

d) the need to consider seasonal variability in occurrence of the target organisms;

e) the number of test results needed for the assessment of relative recovery;

f) the number of laboratories with sufficient capacity and technical expertise available to participate.

It can be acceptable to have a limited number of participating laboratories analysing a wide range of

water types rather than a higher number of laboratories analysing a narrower range of water types

appropriate for the methods being compared.

NOTE Several successful comparisons have been achieved with three to six laboratories. In theory, it is

possible that one laboratory is able to conduct a suitable comparison study provided they have access to a wide

enough range of sample types for which the methods have been characterized.
5.4.2 Number of samples

It is not possible to determine beforehand the exact number of samples required for a valid comparison.

The number depends on the actual difference observed, on the experimental standard deviation and on

the difference considered significant. This International Standard includes an adequacy clause based on

a “predetermined stipulated limit” and the half-width of the confidence interval. If the data are found

inadequate for deciding that the methods are either “different” or “not different”, more samples are to

be collected and examined.

If the methods happen to differ markedly, a small number of samples might suffice to determine this

fact. It is therefore advisable to proceed in stages. The first stage should be planned to detect large

differences between the methods. If large differences are not found (result inconclusive), more samples

are taken until the system is able to detect the average relative difference that corresponds to the

predetermined stipulated limit chosen at the beginning of the trial.
© ISO 2014 – All rights reserved 5
---------------------- Page: 17 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(E)

The total number of samples, n, for a two-sided evaluation (7.2) that would be sufficient for the detection

of a given average relative difference at about 95 % confidence depends on the experimental variance

according to Formula (1):
n= (1)
where
n is the number of samples required for the detection of a difference L;
L is the smallest microbiologically significant mean relative difference;
s is the experimental standard deviation.

EXAMPLE A rather frequently observed value for the experimental standard deviation of the relative

difference is approximately s = 80. In order to detect an average relative difference of 10 % (L = 10 %), n =

25 600/100 = 256 samples is expected to be sufficient for a two-sided evaluation.

For a one-sided evaluation (7.3) the corresponding number of samples can be calculated according to

Formula (2):
n= (2)

The rationale for the derivation of Formulae (1) and (2) is presented in Annex C.

High variability of counts can be experienced due to irregular behaviour of either laboratories or the

range of sample types analysed. Whether it is warranted to continue a comparison of methods study

can be ascertained by an examination of the standard deviation of the mean relative difference. Valid

comparisons are indicated by standard deviations of less than 100.

NOTE With some recent methods based on chromogenic substrates, it is possible to estimate two bacterial

groups simultaneously. One of the groups can be 10 or more times as numerous as the other. It is possible that

the number of samples sufficient for making a final decision of equivalence with the more numerous type is not

sufficient for an organism present in low numbers.
6 © ISO 2014 – All rights reserved
---------------------- Page: 18 ----------------------
SIST EN ISO 17994:2014
ISO 17994:2014(E)
5.4.3 Number of additional samples
Formula (3) can be applied to estimate the number of additi
...

Questions, Comments and Discussion

Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.