Practical Solutions to Traceability and Uncertainty in Accreditation

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Practical Solutions to Traceability and
Uncertainty in Accreditation

• Presented to CITAC-NCSLI Joint Workshop
• Traceability and Uncertainty
• Key Technical Issues and Laboratory
  Accreditation
• PITTCON 2002, New Orleans
• Sunday, 17 March 2002
• Warren Merkel, A2LA Technical Manager

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Presentation Overview

• Relevant ISO/IEC 17025 Requirements
• Accreditation Body Policies
• Practical Approaches for Laboratories
• Future Developments
• Questions

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Measurement Traceability - 17025 Requirements

• Calibrations reference materials traceable to
  SI where possible
• Where not possible, traceable to certified
  reference materials, agreed methods and/or
  consensus standards
• Unless it has been established that the
  associated contribution from the calibration
  contributes little to the total uncertainty of
  the test result

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Traceability- Definition

• Relation to stated references through an unbroken
  chain of comparisons
• Traceability to some stated reference available
  with most RMs
• Normally not SI unit for amount of substance, but
  often other SI units
• All having stated uncertainties
• Little evidence of verification of uncertainty
  claims

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Traceability in Practice

• In many cases, traceability to the test method is
  all that is required by clients
• Uncertainty statements on RMs are important, but
  usually not a significant contributor to
  uncertainty of test
• Performance in proficiency tests can serve as an
  indicator of traceability problems

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Measurement Uncertainty - 17025 Requirements

• 5.4.6.2 - Testing laboratories shall have and
  apply procedures for estimating uncertainty of
  measurement
• Nature of the test method may preclude rigorous,
  metrologically and statistically valid,
  calculation of uncertainty of measurement
• Laboratory shall at least attempt to identify all
  uncertainty components, make a reasonable
  estimation, and ensure that the form of reporting
  of result does not give a wrong impression of
  uncertainty

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Measurement Uncertainty - 17025 Loopholes

• Note 1 - Degree of rigor depends on
• Requirements of the method
• Requirements of the client
• Existence of narrow limits on specification
  conformance
• Note 2 - In cases where a well-recognized test
  method specifies limits to the values of the
  major sources of uncertainty and specifies the
  form of presentation of calculated results, the
  laboratory is considered to have satisfied this
  clause by following the test method and reporting
  instructions

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Measurement Uncertainty - 17025 Reporting
Requirements

• 5.10.3.1 c) Information on uncertainty is needed
  in test reports when it is relevant to the
  validity or application of the test results, when
  a clients instruction so requires, or when the
  uncertainty affects compliance to a specification
  limit
• 5.10.1 - In the case of a written agreement with
  the client, results may be reported in a
  simplified way

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A2LA Interim Policy on Measurement Uncertainty
for Testing Laboratories

• Five categories of test methods
• Intended to facilitate transition- pragmatic
  approach, not ideal in all cases
• Results from review at annual meeting
• Some modification of language
• Publish list of example methods for each category
  by field of testing
• Publish guidance on determining uncertainty in
  testing based on ISO 5725

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A2LA Interim Policy on Measurement Uncertainty
for Testing Laboratories

• Five categories of test methods
• I. Qualitative
• No uncertainty calculations required
• Examples Ignitability Microbiological
  screening
• II. Well-recognized methods that specify limits
  to uncertainty contributions (Note 2)
• No further uncertainty calculations required
• Examples Flash point Hardness
• Problems Modification of method
• What if a client wants uncertainty?

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A2LA Interim Policy on Measurement Uncertainty
for Testing Laboratories

• Five categories of test methods, continued
• III. Published methods that do not specify limits
  to uncertainty sources and/or reporting format
• Uncertainty estimated using standard deviation
  of laboratory control samples
• Examples Alloy analysis by OES VOA
• Problems Normal process for analyzing control
  samples may lead to an underestimate of
  uncertainty
• Quality of control sample

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A2LA Interim Policy on Measurement Uncertainty
for Testing Laboratories

• Five categories of test methods, continued
• IV. Methods requiring identification of major
  uncertainty components and reasonable estimate of
  uncertainty
• Examples PBMS One-off tests
• V. Methods requiring full uncertainty analysis
  consistent with ISO Guide to the Expression of
  Uncertainty in Measurement
• Example Reference material value assignment

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A2LA Interim Policy - Chemical Laboratories

• Majority of methods classified as Category II,
  III, IV
• Classification can vary by laboratory and use
• Use of RMs critical to demonstrating process
  control and evaluating bias
• More practical guidance required

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17025 Requirements Quality Control and
Proficiency Testing

• Laboratory shall have procedures for monitoring
  validity of tests, including
• regular use of CRMs or internal QC using
  secondary reference materials
• participation in interlaboratory comparison or
  proficiency testing programs
• Record data so trends are detectable
• Laboratories in most cases already have
  sufficient data for estimating uncertainty

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Practical Approach - Category II

• Laboratories performing tests in Category II can
  utilize precision data published with method as
  uncertainty estimate if
• Laboratory has data demonstrating that its
  repeatability is comparable to the method data
• Material used for precision estimate is similar
  to materials tested by lab
• Method not modified
• Basis for estimate clearly stated

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Practical Approach - Category III

• Intermediate measures of precision provide
  adequate estimate of uncertainty, if
• Measurement method standardized
• Measurement process is in control
• Control sample well characterized (preferably
  CRM)
• Process for collecting data designed to vary all
  significant uncertainty components

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Steps Laboratories Can Take

• Identify major uncertainty components (App. D of
  EURACHEM/CITAC Guide)
• Establish control charts (ISO 8258)
• Attempt to design QC process to ensure
  representative variation of inputs
• Fewer points ? more emphasis on design
• May need to record additional data related to
  precision as objective evidence

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Steps Laboratories Can Take

• When required to report uncertainty, clearly
  define the basis for the estimate
• Clarify during contract review process
• If possible, determine with client end use of
  data
• Make use of PT study data
• Compare internal precision data to spread of
  results of participants
• If study based on reference value, compare lab
  result/uncertainty with reference

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Future Developments

• Consensus method development
• More rigor in precision data
• Additional detail regarding uncertainty sources
• Accreditation of RM producers
• Increased focus on PT based on reference values
  vs. consensus values
• Increased awareness of traceability uncertainty
  issues in user community

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Conclusions

• Accreditation bodies and laboratories reconciling
  17025 requirements that are ahead of the state of
  development in many industries
• Pragmatic approach to requirements is necessary
• Goal provide data that is fit for purpose

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Contact Information

• Warren MerkelA2LA5301 Buckeystown Pike Suite
  350Frederick, MD, USA 21704
• Direct line 301 644 3204
• Main 301 644 3248
• Fax [phone]
• wmerkel_at_a2la.org
• www.a2la.org