In this first of a series of articles on individual aspects of the assessment of uncertainty of measurement, Stephen MacDonald begins with consideration of the measurand.
Measurement uncertainty (MU) in the context of clinical laboratories is inextricably linked to imprecision. While imprecision is a significant contributor (not to mention easy to determine experimentally), the more abstract concept of the measurand definition is as significant, if not more so, than imprecision. The basis for this is simple. Without defining correctly what we intend to measure, we cannot expect to report a meaningful result.
In many circumstances, incomplete definition of the measurand is the most significant contributor to overall uncertainty. Therefore, we must ensure that it is fully considered in the assessment of MU. Some measurand definitions are very simple, others less so. Importantly, definition of the measurand is required only once. In doing so, we begin the process of modelling our assays. Here, we discuss the impact of an incompletely defined measurand and how we include it in our uncertainty budget by assessing some commonly encountered variables.
Definition of the measurand
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