In some cases, a test may not be defined well enough by the standard, leading to potentially inconsistent application and thus another source of uncertainty. 11 (Issue 2) A Beginner’s Guide to Uncertainty of Measurement Stephanie Bell Centre for Basic, Ther mal and Length Metrology National Physical Laboratory Abstract: The aim of this Beginner’s Guide is to introduce the subject of measurement uncertainty. In particular, the possible consequences and risks associated with a result that is close to the specification limit must be examined. environment. All copyright requests should be addressed to In this case, knowledge of the uncertainty shows whether the result is within the acceptable limits or not. This Guide establishes general rules for evaluating and expressing uncertainty in measurement that can be followed at various levels of accuracy and in many fields — from the shop floor to fundamental research.
The uncertainty may be such as to raise considerable doubt about the reliability of pass/fail statements. All measurements are subject to uncertainty and a measurement result is complete only when it is accompanied by a statement of the associated uncertainty, such as the standard deviation. By international agreement, this uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity value. Repeat the job with as many different rulers as you can find, noting the ruler used each time. This is the combined standard uncertainty multiplied by what is known as a coverage factor. This is derived from statistical theory and the parameter that is determined is the standard deviation.Uncertainty components quantified by means other than repeated measurements are also expressed as standard deviations, although they may not always be characterised by the normal distribution. In ordinary use, the word 'uncertainty' does not inspire confidence. It is a non-negative parameter. Make a note of the measurement, and of the ruler you use.Now do it with another ruler; note the result, and also make a note of which ruler you use. This could give the opportunity to use cheaper, less sensitive equipment or provide justification for extending calibration intervals.Uncertainty evaluation is best done by personnel who are thoroughly familiar with the test or calibration and understand the limitations of the measuring equipment and the influences of external factors, e.g. The implications of that risk may vary considerably. It is evaluated by combining a number of uncertainty components. Are the measurements all the same?Now ask colleagues to do the same, noting the measurements for each ruler. However, when used in a technical sense it carries a specific meaning. In many tests there will be uncertainty components that need to be evaluated on the basis of previous data and experience, in addition to those evaluated from calibration certificates and manufacturers specifications.Accreditation bodies are responsible for ensuring that accredited laboratories meet the requirements of ISO/IEC 17025. JCGM 104 – Evaluation of measurement data – An introduction to the "Guide to the expression of uncertainty in measurement" (ISO/IEC Guide 98-1) (For the moment, only JCGM 100 is available in HTML ; PDF versions of JCGM 100, 101 and 104 are available here) The following parts are under preparation:
In metrology, measurement uncertainty is the expression of the statistical dispersion of the values attributed to a measured quantity. ISO/IEC Guide 98-3:2008 is a reissue of the 1995 version of the Guide to the Expression of Uncertainty in Measurement (GUM), with minor corrections.
the formal ISO Guide principles set out in the ISO Guide to the Expression of Uncertainty in measurement [H.2]. The evaluation of repeated measurements is done by applying a relatively simple mathematical formula. The measurement un Repeat the job with as many different rulers as you can find, noting the ruler used each time. Repeated measurements will show variation because of these factors. This can involve much time and effort for all concerned parties, which in many cases could have been avoided if the uncertainty of the result had been known by the customer.Consequently, calibration laboratories are used to evaluate and report uncertainty. The process of evaluation highlights those aspects of a test or calibration that produce the greatest uncertainty components, thus indicating where improvements could be beneficial.