Uncertainty of Measurement: what it means
All measurements, be they chemical, microbiological or physical, carry some uncertainty. Testing laboratories have, by necessity, become familiar with the concept of measurement uncertainty. Their clients – who must interpret and act on the basis of uncertain measurements – may be less familiar. The intention of this fact sheet is to explain why it is necessary to understand Measurement Uncertainty, the basic fundamentals of what it is, how its calculation can differ between laboratories, and its practical implications. It thus complements our guideline on microbiological measurement uncertainty, which is primarily aimed at testing laboratories. We also offer bespoke training in this.
Since 1999 ISO 17025 "General requirements for the competence of testing and calibration laboratories" has required that testing laboratories "shall have and shall apply procedures to estimate the uncertainty of measurement" (ISO 1999;ISO 2005). At Campden BRI we frequently advise clients' laboratories on this.
Until quite recently – at least in the food and drinks industry – most users of laboratory results have been unconcerned by measurement uncertainty, leaving it as a technical topic to be handled by their laboratories. However, ISO 17025 requires laboratories to accompany a result with a measurement uncertainty estimate:
This means you may be presented with uncertainty on a result even when you haven't asked for it, like this example:
Ochratoxin A = 2.07 μg/kg ± 0.47 μg/kg*
* The reported expanded uncertainty is based on a standard uncertainty multiplied by a coverage factor of k = 2 to give a confidence level of approximately 95%
The reported result (e.g. 2.07) is the laboratory's best estimate of the 'true' value. But no one expects it to be exactly correct, raising the question of what the 'true' value might be. The definitive "Guide to the expression of uncertainty in measurement", known as "the GUM" (Joint Committee on Guides in Metrology 2008) studiously avoids the idea of a true value, defining uncertainty of measurement as a "parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand". But, at least informally, we can consider the 'expanded uncertainty' (e.g. ± 0.47) as a reasonably credible range for the true value.
This short note discusses the interpretation of such uncertain results under two headings:
'Quality' of the result
It is tempting to see Uncertainty of Measurement as failure to properly perform a test, and smaller stated measurement uncertainty (MU) as evidence of better performance. But it's not as simple as that…
The definition of MU (above) contains that slippery word "reasonably". Different people can attribute different values, all reasonable, giving different stated MUs for the same performance.
This was recognised as long ago as 2004: "the current lack of standardisation has caused the estimated measurement uncertainties to vary widely, even for the simplest determinations" (Visser 2004). Solfrizzo et al. (2009) recognised that "a large variability of measurement uncertainty … was probably due to non-harmonized interpretation of the term". Very recently (Breidbach et al. 2010) noted inconsistent MU estimations and that "Clients will more likely than not choose laboratories with narrower MU", even though narrower MUs might reflect less stringent MU evaluation rather than better measurement.