Key Topics

What is measurement uncertainty?

Whenever a measurement is made, there will always be some uncertainty or doubt about the result obtained. This is not because the analyst has made a mistake in the application of the measurement method; all measurements are subject to errors which will contribute to the uncertainty in the result.

The ISO definition of uncertainty is, ‘A parameter associated with the result of a measurement, that characterises the dispersion of results that could reasonably be attributed to the measurand.’

The key points in the definition are:

• Measurement uncertainty applies to the result of a measurement. There will be a number of factors associated with the method of measurement used that will contribute to the uncertainty in the result. These include uncontrollable random variations in the measurement process, limitations in the measuring equipment used and uncertainties in reference values used for calibration.
• Measurement uncertainty describes a range of values. If the uncertainty has been evaluated correctly, the range should include the true value for the measurand (note that the ‘measurand’ is simply the quantity or property being measured). For example, if the concentration of lead in a sample of soil is reported as 95±14 mg kg^-1, the result should be interpreted as, ‘The true value for the concentration of lead in the soil sample is somewhere between 81 mg kg^-1 and 109 mg kg^-1’.
• The uncertainty estimate should characterise the dispersion of results that could reasonably be attributed to the measurand. It should therefore take account of all the significant sources of variability in the measurement results and must include the effect of both random and systematic errors.