Measurand

The measurand is the quantity intended to be measured, and its definition is a prerequisite for any meaningful measurement. The VIM (3rd edition, 2012) provides the formal definition, noting that the measurand may be a property of a phenomenon, body, or substance, and that it must be specified with sufficient detail to avoid ambiguity. For example, the measurand could be the “length of a given rod at 25 °C” rather than simply “length,” because the rod’s length varies with temperature. The concept is central to the Guide to the Expression of Uncertainty in Measurement (GUM), which requires that the measurand be defined clearly to evaluate measurement uncertainty. In practice, the measurand is often an idealized quantity, such as the “true” value, which is unknowable but approximated by measurement results.

The measurand is often confused with the quantity being measured in a practical sense, but the distinction is crucial. The measurand is the intended target, while the “quantity value” is the result of measurement, and the “indicated value” is what the instrument displays. For instance, when weighing an object, the measurand is the mass of the object, but the measurement result may be affected by buoyancy, so the measurand must be defined to include corrections. The measurand is also distinct from the “influence quantity,” which is a quantity that affects the measurement but is not the target, such as ambient temperature. This distinction is essential for uncertainty analysis, as influence quantities contribute to the uncertainty of the measurand’s estimate.

The definition of the measurand directly impacts the evaluation of measurement uncertainty. According to the GUM, the uncertainty of a measurement result is an expression of the doubt about the value of the measurand, and it is evaluated by considering all sources of variability, including the definition of the measurand itself. If the measurand is poorly defined, the uncertainty may be underestimated or overestimated. For example, measuring the thickness of a paint layer requires specifying whether the measurand is the average thickness over a given area or the thickness at a specific point. The GUM recommends that the measurand be described by a function of other quantities, allowing the uncertainty to be propagated through the measurement model.¹ This approach is fundamental to calibration and metrological traceability.

Beyond the basic definition, the measurand concept has subtle implications. In chemical metrology, the measurand may be the amount-of-substance concentration of a specific analyte, but the definition must account for the matrix and the analytical method, leading to the concept of “operational measurand.”² In dynamic measurements, the measurand may vary with time, requiring a definition that includes the time window or frequency band. The VIM also notes that the measurand can be a “nominal property,” such as the color of a solution, which is not a quantity but is still treated as a measurand in some contexts. Historically, the term was formalized in the 1984 edition of the VIM, but the concept has roots in the work of early metrologists like Maxwell, who emphasized the importance of defining the quantity to be measured.³ These nuances are critical for international comparisons and for ensuring measurement results are comparable across laboratories.

VIM

International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM), a joint publication of BIPM, IEC, IFCC, ILAC, IUPAC, IUPAP, and OIML.

GUM

Guide to the Expression of Uncertainty in Measurement, a document that establishes general rules for evaluating and expressing measurement uncertainty.

Influence quantity

A quantity that is not the measurand but that affects the result of the measurement.

Traceability

The property of a measurement result whereby it can be related to a reference through a documented unbroken chain of calibrations.