National Physical Laboratory

Uncertainty Guides

A Beginner's Guide to Uncertainty of Measurement
Measurement Good Practice Guide No 11

A gentle and short introduction to uncertainty of measurement for beginners, including laboratories preparing for UKAS accreditation. The guide explains the concept and importance of measurement uncertainty, using examples from everyday life. It illustrates how to estimate uncertainties in real measurement situations, showing a detailed uncertainty calculation step by step.

The aim of the Beginner's Guide is to introduce the subject of measurement uncertainty. Every measurement is subject to some uncertainty. A measurement result is only complete if it is accompanied by a statement of the uncertainty in the measurement. Measurement uncertainties can come from the measuring instrument, from the item being measured, from the environment, from the operator, and from other sources. Such uncertainties can be estimated using statistical analysis of a set of measurements, and using other kinds of information about the measurement process. There are established rules for how to calculate an overall estimate of uncertainty from these individual pieces of information. The use of good practice - such as traceable calibration, careful calculation, good record keeping, and checking - can reduce measurement uncertainties. When the uncertainty in a measurement is evaluated and stated, the fitness for purpose of the measurement can be properly judged.

Estimating Uncertainties in Testing
Measurement Good Practice Guide No 36

This guide follows on from A Beginner's Guide to Uncertainty of Measurement, developing treatment of the subject to a level intended for testing laboratories. It presents principles and guidance for the estimation of measurement uncertainty which are applicable to most fields of testing but is not intended for calibration laboratories, which normally need to have a greater depth of knowledge of these principles than is presented here.

The principles and guidance given here are consistent with the Guide to the Expression of Uncertainty in Measurement, first published in 1993 by ISO, Geneva, and with the requirements of ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories where they relate to estimating and reporting uncertainty of measurements.

Metrology - In Short

A brief introduction to the science of measurement and the international measurement community.

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