9.10.4 Measurement uncertainty

There is an inherent level of uncertainty associated with the measurement of water quality characteristics, in addition to the uncertainty arising from sampling. This inherent uncertainty arises from a number of sources, but it primarily relates to the accuracy of the laboratory equipment used to produce a result, and various measurement errors that may be introduced through the analytical process.

In some cases, the level of uncertainty will be insignificant relative to the quoted result; in other cases, however, it can be quite significant. Under the ISO Standard used by NATA for accreditation, AS ISO/IEC 17025—2005 – General Requirements for the Competence of Testing and Calibration Laboratories, laboratories are required to estimate the uncertainty associated with the results they produce (known as the measurement uncertainty).

Upon request, NATA-accredited laboratories are required to provide the measurement uncertainty (MU) associated with a particular analytical result (x), which should be expressed by the laboratory as either x ± MU, or as a percentage of x, or in any other manner accepted by the relevant accreditation body.

Organisations performing water quality testing are encouraged to request that their laboratories provide MU data as part of their analytical results reporting. This will promote an appreciation of the variability in the analytical data being received.