iso guide 35 statistical procedures for reference materials

8/9/2019 ISO Guide 35 Statistical Procedures for Reference Materials

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ISO Guide 35:Current requirements and proposed

revision

HKAS Training Course21 January, 2014

Daniel Tholen, M.S.


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ISO Guide 35 Training

ISO Guides for Reference MaterialsGuide 34 requirements in Guide 35

– Traceability, Characterization,

– Homogeneity, Stability – Uncertainty

Guide 34 requirements not in Guide 35

– Replacement batches – Qualitative properties

– Non-certified Reference Materials

– Uncertainty in the presence of degradation


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ISO Guide 35 Training

Guide 35 clarifications – Use of previous batches for homogeneity and

stability

– Minimum uncertainty for homogeneity andstability

– Independent datasets for characterization

– Short term stability and transportation


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ISO Guides for Reference Materials

Guide 30: terms and definitions for RM – 1992 version, amended 2008

Guide 31: contents of certificates and labels

Guide 34: General requirements forcompetence of RM producers

Guide 35: General and statistical principles

for certification – Tentative Revision Title: General guidance for

the assignment of property values


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ISO Guides developed out of step – Guide 30 1981 1992 (amended 2008)

– Guide 31 1981 2000

– Guide 34 1996, 2000, 2009

– Guide 35 1985, 1989, 2006 2014?

Each revised document slightly changes

others, especially Guides 34 and 35


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Revisions to ISO Guides 30 and 31 – 2014 ?

New Guidance documents:

– Internal production of RMs for QC (Guide 80) – RMs for Nominal properties (TR 79)

– Guidance for use of RMs (Guide 33)

ISO 17034 ?

Next meeting of REMCO in July, USA


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Guide 35 references in Guide 34

Guide 35 is a normative reference for Guide 34 – “…indispensible for the application of this document.”

4.1.2 Quality policy - The quality policy shall

include but shall not be limited to the followingcommitments:

– b) to produce, where applicable, certified reference

materials according to the requirements of ISO Guide

35 and accompanied by certificates meeting therequirements of ISO Guide 31;


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Guide 35 references in Guide 34

Summary: Guide 35 is referenced for:

– 5.12 Metrological traceability

– 5.13 Assessment of homogeneity

– 5.14 Assessment of stability – 5.15 Characterization

– 5.16 Assignment of property values and their

uncertainties


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Current Approach in Guide 35

Intended for REMCO members who are

responsible for transferring traceability

Assumes high level technical competence

by RMP, including statistical techniques

Requirements are not clearly stated

– Not an assessment document


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Current Approach in Guide 35

Not designed for situation of commercial

Reference Material Producers

– Cost considerations

– Time to market

– Uncertainty

Difficult for many Accreditation Bodies

– Statistics, traceability, uncertainty


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Issues not addressed in Guide 35

Non-certified RMs (RMs for use other than

calibration) – e.g. QC and PT

Demonstrating equivalence of replacement

batches

– Use of information from similar batches

Uncertainty in the presence of degradation

Qualitative RMs

Stability under usage conditions

– Might not be addressed in revision


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Confusing topics in Guide 35

Minimum uncertainty for homogeneity ?

– Must be > 0

Minimum uncertainty for stability ?

– Must be > 0

Short term stability vs. transportation

Characterization in one laboratory using asingle (primary) method


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Current status of revision

Four sections discussed

– Draft revisions prepared for conference calls

– Many issues discussed on conference calls

– No revisions circulated after conference calls

Draft CD to be distributed for comment,

February, 2014

Comments discussed in July, 2014

Approval time?


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Proposed changes from latest

circulated drafts

General changes

– Considerations for commercial RMPs

– Use of information from previous batches

– Restructure for process of production

Design

Homogeneity

StabilityCharacterization

Uncertainty

Statistical Annex

Examples


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Draft Scope

This Guide gives general guidance and explains

concepts to assist with the understanding and

development of valid methods to assign values to

properties of a reference material, including theevaluation of their associated measurement

uncertainty, and the establishment of their

metrological traceability.


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Draft Scope (continued)

This Guide complements ISO Guide 34 by

providing detailed descriptions of acceptable

approaches for the production of reference

materials with reference to the assessment ofhomogeneity and stability, the characterisation of

reference materials, estimating uncertainties of the

assigned values and by giving information on how

to achieve and demonstrate the metrologicaltraceability of assigned values.


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Homogeneity –

General considerations

Homogeneity testing is usually necessary

– New Reference materials

– Inherently inhomogeneous materials (e.g., food

matrix, soils, gases)

Can use estimates of uncertainty from

previous batches of similar material

– With confirmation check


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Homogeneity –

Uncertainty


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Homogeneity – Minimum

Number of Units to Test


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Homogeneity – Other

considerations (in current G35)

Check within-unit homogeneity

Check for trend in testing orderCheck for trend in production order

Check for outlier difference between replicates

Can use interlaboratory nested designReport minimum sample amount (uptake)

Do not need to test all properties


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Stability –

General considerationsCan use information from previous batches

of similar material

– Need technical justification

Can allow ults = 0

– Need evidence from previous batches

Should test all properties


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Stability –

Post-release monitoringFrequent monitoring can be used when

there is little classical or accelerated stability

testing

No monitoring needed if shelf life is short

Possible change between monitoring points

less than 1/3 claimed uncertainty

If there are replacement batches, there

should always be testing of retained

samples at time of expiration


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Stability –

Short term StabilityShort term Stability includes 2 factors

– Transportation

– Storage by user under alternative conditions

If no alternative storage conditions, use

‘uncertainty due to transport’ (utran) instead

of u sts

Should generally test all properties


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Stability – Types of

Long-term StudiesClassical, or real-time studies

– Analyses as time elapses

– Includes reproducibility variance

Isochronus studies

– All analyses at same time (no reproducibility)

– Requires conditions under which no

degradation occurs

Accelerated studies

– Requires model for degradation

– Requires inclusion of uncertainty of model


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Stability – Uncertainty when

there is degradationWhen predicted change exceeds 30% of

utarget

Need to report change and uncertainty ofchange (from Guide 34)

Report time-dependent property value

Report time-dependent uncertaintyCould report as equations or as property

value and uncertainty at point of sale.


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Stability – Usage conditions

Applies when unit of RM can be repeatedly

subsampled within claimed shelf life

Mentioned as a consideration, no

procedures


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Characterization - General

Four types of studies (as discussed in G34)

a. Single (primary) method in single laboratory

b. Two or more methods in one or more

laboratories

c. One or more methods in a network of

laboratories

d. Method-specific, operationally definedproperty values, using network of laboratories


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Characterization - proposed

Three types of studies (combine b&c)

a. Primary method in single laboratory

- Advisable to have independent confirmation method

b. Two or more methods in one or more

laboratories

- Need 5 or more independent data-sets

c. Method-specific, operationally defined propertyvalues, using network of laboratories


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Characterization –

unresolved issuesWhat is a primary method? – “…a method having the highest metrological properties,

whose operation can be completely described and

understood, for which a complete uncertainty statementcan be written down in terms of S.I. units” (CCQM)

Combining approaches b & c

Use of robust statistical procedures

Use of same study to characterize RM and

to demonstrate competence of laboratory


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Characterization – special issues:

Purity CRMs

Purity

– Determined directly

– Determined by subtracting impurities from 100%

– Requires evidence of identity


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Secondary CRMs

A material whose values are assigned by

directly comparing candidate CRM with an

already characterized CRM

– Reference and candidate must be comparable

– Reference must have a link to a primary CRM

Therefore tertiary CRMs can be linked to secondary

– Method for validation meets traceability criteria

Then CRM can be characterized in a single

laboratory, using a non-primary method


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Identity CRMs

Identity is usually not a measurement result,

but is a conclusion drawn on measurement

results from one or several methods

– e.g., DNA sequence

Useful only if error probability is negligible

Slight inhomogeneity and instability should

not change the conclusion

Requires information on the source of the

material and processing steps


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Presence/Absence CRMs

A quantitative measurement is evaluated in

a qualitative manner

– e.g., value above or below a limit of detection

Advisable to test with all relevant methods

Uncertainty should state the confidence

level of the limit of detection


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Ordinal properties

Quantities where the outcome of

measurement puts the result in a certain

ordered class

CRMs will be useful only if measurement

places the value unequivocally

– All technically accepted measurements by all

participating labs place material in same class


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Qualitative properties

Properties such as color, order, shape, etc.

– In some cases these properties have

quantitative forms

Usually a method-defined measurand


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Uncertainty

There are some clarifications to current

requirements, but no substantive changes.


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Thank you

iso guide 35 statistical procedures for reference materials

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