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Standard Interpretation



SIR #: 66
Standard: 2003 NELAC
Section: 5.5.4.6.1
Link to relevant standard

REQUEST:

Please explain what types of procedures for estimating uncertanity of measurements. I am not sure which area you mean. Thank You

TNI FINAL RESPONSE:

(Quality System Expert Committee/NELAP Accreditation Council, x-x-10)

The following is a previously issued statement regarding uncertainty (found on the TNI Website - Previous Interpretations): Uncertainty Clarification - Section 5.5.4.6 "Estimation of Uncertainty of Measurement" has created some confusion. Please note that as a laboratory it is impossible for you to calculate "Total Uncertainty" unless you are given all of the additional pieces from external sources to the lab itself. This section is intended to advise a laboratory to have a "Procedure on Uncertainty for the Laboratory Portion" in place, so that if requested by a client it could be determined. The key language within this section can be found in Section 5.5.4.6.2, " ... In certain cases the nature of the test method may preclude rigorous, metrologically and statistically valid, calculation of uncertainty of measurement. In these cases the laboratory shall at least attempt to identify all the components of uncertainty and make a reasonable estimation, and shall ensure that the form of reporting of the result does not give a wrong impression of the uncertainty. ..."

The laboratory can arrive at an estimation of uncertainty through a review of quality control data. For example, a comparison of CCV data and LCS data could lead to an estimate of the uncertainty caused by the preparation process. A similar comparison between LCS data and MS data could lead to an estimate of the uncertainty introduced by the sample matrix. Organizations (such as A2LA and the Department of the Navy) have developed tools to aid a laboratory in accomplishing this task.