Traceability & Uncertainty of Measurement

Below is a summary of L-A-B Policy 001 which defines the way in which laboratories will prove their Traceability of Measurements, and express their Measurement Uncertainty.  Please refer to L-A-B Policy 001 for the exact requirements of Laboratory Accreditation Bureau.

• Traceability

• Demonstration of Traceability

• Measurement Uncertainty

• Best Measurement Capability

• Calibration and Dimensional Inspection Laboratories

• Testing Laboratories

• Requirements for Reporting Measurement Uncertainty in Testing Labs

• Needs Assessment

Traceability

1. The calibration program shall assure traceability of measurements, and/or verification and validation of equipment is traceable, wherever possible, to NIST or other National Measurement Institute (NMI) is to be captured within L-A-B Form 001 - Traceability Tracking.
   
   Calibration certificates, where applicable, shall indicate the traceability to an NMI or intrinsic standard, the measurement result and the associated uncertainty of the measurement and/or a statement of compliance with an identified metrological specification. To ensure actual traceability, the path of reference standard verification back to the NMI shall be clear. The evidence of the investigation of the path back to the NMI shall be available for verification by the L-A-B assessors. The requirements for a primary reference, transfer, and working standards or reference materials shall be defined by the laboratory. When defining those requirements the laboratory shall identify the critical characteristics that may affect the traceability for the calibration and/or test. Those characteristics include the requirements stated in ISO/IEC 17025. Critical characteristics may include handling, reporting, equipment, and methodology using the standards etc. Depending on the level of standard and the frequency of use, transport, ownership, etc. the laboratory shall apply the appropriate degree of procedural control.
   
   Where intrinsic standards are used, the laboratory should demonstrate by measurement-assurance techniques, interlaboratory comparison, or other suitable means that its intrinsic-measurement results are correlated with an NMI.
    

2. When traceability to an NMI is not possible, the laboratory should have a procedure that will provide satisfactory evidence that the results are correlated, for example by participation in a suitable interlaboratory comparison or proficiency testing. Other satisfactory evidence would be an internationally accepted standard in the field concerned; suitable reference material; ratio or reciprocity-type measurements; or mutual consent standards that are clearly specified and mutually agreed upon by all parties concerned.

Demonstration of Traceability

1. L-A-B prospective clients and accredited labs may submit appropriate physical standards and test and measurement equipment (M&TE) directly to NIST or when appropriate, to another national metrology institute (NMI). Accredited laboratories may obtain certified reference materials from NIST (called Standard Reference Materials (SRM) under copyright) or from another NMI. Use of an NMI other than NIST must be documented as the appropriate NMI relevant for the scope of accreditation and stated uncertainties.

2. Testing laboratories that perform calibrations only for themselves do not need to be accredited as a calibration laboratory. Calibration laboratories that perform specific calibrations only for themselves to support their accredited services do not need to be accredited for those calibrations. For the purpose of assuring traceability, an accredited laboratory may calibrate its own equipment if the appropriate requirements of L-A-B and 17025 have been met. The laboratory shall demonstrate is competency to perform the calibrations it undertakes.

3. L-A-B prospective clients and accredited labs may use a State Weights and Measure lab that is recognized by the NIST Office of Weights and Measures (OWM) State Laboratory Program and has a current “Certificate of Measurement Traceability” issued by the Weights and Measures Division of NIST. This certificate must be available during the assessment of the laboratory.

4. L-A-B Accredited Laboratories that do not demonstrate traceability as defined in 1, 2 or 3 above, shall use accredited calibration laboratory services wherever available. Accredited calibration laboratories are those accredited by L-A-B or an accreditation body that is recognized as a signatory of the Asia Pacific Laboratory Accreditation Cooperation “APLAC” MRA and/or International Laboratory Accreditation Cooperation “ILAC” MRA. A listing of L-A-B accredited laboratories is available on our website at www.l-a-b.com. When utilizing accredited calibration laboratory services, the calibration certificates shall be accompanied by a recognized accreditation body symbol or otherwise make reference to accredited status to be considered satisfactory for traceability purposes.

5. If a L-A-B applicant or accredited laboratory submits physical standards or M&TE to a calibration provider that is not accredited by L-A-B or other L-A-B acceptable laboratories, the laboratory shall:

   1. Document that an appropriate accredited calibration provider is not available.  Documented evidence is required including results of searches of appropriate AB’s websites.

   2. Audit the claim of traceability of the provider of the calibration service and document the following areas related to the calibration and claim of traceability of its standards and M&TE:

      1. Information regarding assessment of the quality system used by the calibration service provider.  The information must include details on an assessment of the calibration service provider.  The assessment can be done by the laboratory (if a qualified person is on staff) or another AB.

      2. The calibration procedure(s) used by the calibration service provider.

      3. The physical standards or other M&TE used by the calibration service provider (including evidence of traceability to standards maintained by NIST or an appropriate NMI and copies of relevant calibration certificates).

      4. Information regarding the calibration intervals of relevant standards or other M&TE.

      5. The environmental conditions of the laboratory.

      6. The method(s) by which uncertainties are determined e.g., ISO Guide to the Expression of Uncertainty in Measurement (GUM).

      7. The relative uncertainties achieved at all steps of the process.

   3. Pursue the traceability chain until traceability to appropriate stated references is completely validate, when a calibration service provider submits physical standards and/or M&TE used in the calibration to another laboratory(s) not accredited by L-A-B or L-A-B acceptable accreditation body.

   4. Enter the audit documentation, including all findings of noncompliance and resolutions of those findings, into the laboratory’s quality management record system.

   5. If traceable calibration is not available or appropriate, laboratories may demonstrate comparison to a widely used standard that is clearly specified and mutually agreeable to all parties concerned, particularly in measurements where NIST does not maintain a US national standard.  For example, NIST does not maintain a standard for all hardness testing scales.  There are several widely used commercial standards available for hardness.  However, these standards may not all give equivalent measurement results; therefore, it is important to specify which standard is used and to obtain agreement among all parties involved that the choice made is acceptable

Measurement Uncertainty

L-A-B has created a new uncertainty Budget Checksheet to help you through the steps of creating an uncertainty budget.

• (Form 230) - Uncertainty Budget Checksheet (New)

1. Uncertainty of measurement comprises, in general, many components.  Some of these components may be estimated on the basis of the statistical distribution of the results of series of measurements and can be characterized by experimental standard deviations, and are called Type A evaluation.  The Type A can be applied when several independent observations have been made for one of the input quantities under the same conditions of measurement.  If there is sufficient resolution in the measurement process, there will be an observable scatter or spread in the values obtained.  In this case, the standard uncertainty is the experimental Standard Deviation of the mean that follows from an averaging procedure or an appropriate regression analysis.

2. Estimates of other components can only be based on experience or by scientific judgment based on all available information on the possible variability of the measurement and other information, and are called    Type B evaluation. Values belonging in this category may be derived from:

   1. Previous measurement data.

   2. Experience with or general knowledge of the behavior and properties of relevant materials and instruments.

   3. Manufacturer’s specifications.

   4. Data provided in calibration and other certificates.

   5. Uncertainties assigned to reference data taken from handbooks.

   3.  The measurement uncertainty for a given calibration is the combination of
        all the Type A and Type B components of the uncertainty budget.

Best Measurement Capability

Best measurement capability (always referring to a particular quantity, viz. the measurand) is defined as the smallest uncertainty of measurement that a laboratory can achieve within its scope of accreditation, when performing more or less routine calibrations of nearly ideal measurement standards intended to define, realize, conserve or reproduce a unit of that quantity or one or more of its values, or when performing more or less routine calibrations of nearly ideal measuring instruments designed for the measurement of that quantity. The assessment of best measurement capability of accredited calibration laboratories is based on the method described in this document, and shall be supported or confirmed by experimental evidence.

1. L-A-B requires that calibration accredited laboratories state on their Scope of Accreditation an expanded uncertainty of measurement U, obtained by multiplying the standard uncertainty u(y) of the output estimate by a coverage factor of k,
   
   U=ku(y)
   
   In cases where a normal (Gaussian) distribution can be attributed to the measurand and the standard uncertainty associated with the output estimate has sufficient reliability, the standard coverage factor k=2 shall be used. The assigned expanded uncertainty corresponds to a coverage probability of approximately 95%. These conditions are fulfilled in a majority of cases encountered in calibration work.

Calibration and Dimensional Inspection Laboratories

1. Calibration and dimensional inspection laboratories shall report their measurement uncertainty on all calibration certificates, and inspections reports unless it can be proven that the client does not want it reported.  Evidence that the client does not want the calibration and dimensional inspection uncertainty reported shall be available for an assessor to review at the time of an assessment.  Regardless of whether the client wants the measurement uncertainty reported, the laboratory shall retain sufficient information to report the uncertainty. 

2. Laboratories may issue certificates with a statement of compliance (or conformance to a specification). When the laboratory issues a statement of compliance it must ensure that:

   1. The specification is a national / international standard or one that has been provided or agreed to by the customer.

   2. The measurements needed to determine conformance are within the scope of accreditation.

   3. When the measurement result is determined to be within a specified tolerance, the associated uncertainty of the measurement result is properly taken into account with respect to the tolerance by a documented procedure or policy. The policy or procedure must be established and implemented by the laboratory that defines the decision rules used by the laboratory for declaring in or out of tolerance conditions.

      1. The default decision rule is found in ILAC-G8:1996, Guidelines on Assessment and Reporting of Compliance with Specification, section 2.5. With agreement from the customer, other decision rules may be used as provided for in this section of the Requirements.

   4. The certificate relates only to metrological quantities and states which clauses of the specification are certified to have been met.
       

3. The calibration laboratory shall report its best measurement capability, as defined above, on its proposed Scope of Accreditation.

4. L-A-B does allow the best measurement capability to be reported on the appropriate calibration/dimensional inspection certificate only if the best measurement capability was computed (and reported on the scope) at the fullest extent of the measuring range representing the worst capability of the lab to perform that measurement. Other capabilities (uncertainties) that may be worse than the one reported on the scope of accreditation shall be reported on the certificates of calibration or dimensional inspection if they more appropriately represent the measurement taken.

Testing Laboratories

1. The complexity involved in estimation of uncertainty of measurement in the case of testing varies considerably from one test field to another and also within one field itself. A less metrologically rigorous process than that which can be followed for calibration can also often be used. Clause 5.4.6.2 of ISO/IEC 17025 allows for these factors. The degree of rigor needed in an estimation of uncertainty of measurement depends on factors such as:

   1. Requirement of the test method

   2. Requirement of the client

   3. There are narrow limits on which decisions conformance to a specification are based

2. If the test method is well recognized (ASTM, ISO) and specifies limits to the values of the major sources of uncertainty of measurement, 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.

3. Uncertainty components/budgets are a combination of many factors that may include, but are not limited to:

   1. Reference standards

   2. Reference materials

   3. T/C methods used

   4. Equipment used

   5. Environmental conditions,

   6. Properties and condition of item being tested

   7. Calibration

   8. Operator

   9. Known physical characteristics of components such as, coefficient of thermal expansion. These often can be looked up in engineering and scientific handbooks.

Requirements for Reporting Measurement Uncertainty in Testing Labs

1. The laboratory shall perform and have available for the assessor a Needs Assessment, as defined below. They shall also produce a procedure(s) for calculating their measurement uncertainty, which includes creation of an uncertainty budget for the assessor to review during your assessment or surveillance visit.

2. All necessary measurement uncertainties, as defined in the Needs Assessment, shall be available for review by the assessor during the assessment or surveillance visit. In the event that the calculations and uncertainty budgets are not available, the laboratory runs the risk of having those tests, for which the uncertainty is missing, removed from their Scope of Accreditation.

3. Laboratories applying for accreditation during the above periods shall have the appropriate documentation that relates to the dates defined above.

4. Currently L-A-B does not require reporting of measurement uncertainty on Test Reports, unless it is required/requested by the laboratory’s client.

Needs Assessment

The laboratory shall do a Needs Assessment for all tests on their proposed Scope of Accreditation in the “Remarks” column for each “Field of Testing”. The Needs Assessment shall define what actions will be necessary with regard to the reporting of uncertainty. The Needs Assessment matrix shall group the tests into five categories as defined below.

1. Qualitative or semi-quantitative tests that require no uncertainty budgets

2. A test performed to well-recognized test methods that specify limits to the values of the major sources of uncertainty of measurement and specifies the form of presentation of calculated results. These are defined in ISO/IEC 17025 Clause 5.4.6.2 Note 2.

3. Chemical, environmental, or biological test methods based on published regulatory or consensus methods; such as, FDA, EPA, AOAC, and ASTM, for which uncertainty is not defined in the method. For these types of test, uncertainty can be estimated using the standard deviation of laboratory control samples for more than 50 points. (This does not include laboratory-developed methods that require validation and are covered below).

4. Test methods that need identification of the major components of uncertainty and a reasonable estimate of the measurement uncertainty.

5. Test methods that need identification of all components of uncertainty and detailed measurement uncertainty budgets calculated in accordance with published methods that are consistent with the ISO “ Guide to the Expression of Uncertainty of Measurements”