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Measurement Uncertainty

UncertaintyUncertainty of Measurement

Knowledge of the uncertainty of measurement of testing and calibration results is fundamentally important for laboratories, their clients and all institutions using these results for comparative purposes. Competent laboratories know the performance of their methods and the uncertainty associated with the results. Uncertainty of measurement is a very important measure of the quality of a result or a testing method. Other such measures are reproducibility, repeatability, robustness and selectivity.

What is Uncertainty of Measurement?

According to the International Vocabulary of Metrology (VIM) 2.26, a non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used.

  • Measurement uncertainty includes components arising from systematic effects, such as components associated with corrections and the assigned quantity values of measurement standards, as well as the definitional uncertainty. Sometimes estimated systematic effects are not corrected for but, instead, associated measurement uncertainty components are incorporated.
  • The parameter may be, for example, a standard deviation called standard measurement uncertainty (or a specified multiple of it), or the half-width of an interval, having a stated coverage probability.
  • Measurement uncertainty comprises, in general, many components. Some of these may be evaluated by Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values from series of measurements and can be characterized by standard deviations. The other components, which may be evaluated by Type B evaluation of measurement uncertainty, can also be characterized by standard deviations, evaluated from probability density functions based on experience or other information.

In general, for a given set of information, it is understood that the measurement uncertainty is associated with a stated quantity value attributed to the measurand. A modification of this value results in a modification of the associated uncertainty.

 

What is to be considered in an uncertainty of measurement budget?

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

  • Reference standards
  • Reference materials
  • T/C methods used
  • Equipment used
  • Environmental conditions
  • Properties and condition of item being tested
  • Calibration
  • Operator
  • Known physical characteristics of components such as, coefficient of thermal expansion

 

How uncertainty is applied to a measurement when making a statement of compliance (ILAC G8)

  • If the specification limits are not breached by the measurement result, extended by half of the expanded uncertainty interval at a level of confidence of 95%, then compliance with the specification can be stated.
  • Where an upper specification limit is exceeded by the measurement result evens when it is extended downwards by half of the expanded uncertainty interval, then noncompliance with the specification can be stated.
  • If a lower specification limit is breached even when the measurement result is extended upwards by half of the expanded uncertainty interval, then non-compliance with the specification can be stated.
  • If the measured single value without the possibility of measuring more samples from the same unit of product falls sufficiently close to a specification limit, such that half of the expanded uncertainty interval overlaps the limit, it is not possible to confirm compliance or non-compliance at the stated level of confidence. The measurement result and expanded uncertainty should be reported together with a statement indicating that neither compliance nor non-compliance was demonstrated. A suitable statement to cover these situations would be, for example:

    If the measurement result is exactly on the specification limit, it is not possible to state compliance or non-compliance at the stated level of confidence. The test result and expanded uncertainty should be reported together with a statement indicating that neither compliance nor non-compliance was demonstrated at the stated level of confidence.

 

Reporting the Uncertainty of Measurement

When reporting the measurement and its associated uncertainty, the certificate / report may include the contributors of uncertainty or a record within the quality management system shall be present for the identification of the contributors that affected the measurement taken. In many cases, reporting the expanded uncertainty in a certificate or report is more than likely to be expressed at a coverage level of k = 2 and a confidence level of approximately 95%. Reporting the uncertainty of measurement is also critical in maintaining traceability. Without the uncertainty reported on the certificate / report, the chain of traceability is broken and confidence cannot be maintained in the measurement taken.

Measurement Uncertainty Budget Calculator