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In metrology (the science of measurement), a standard (or etalon) is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. [1] Standards are the fundamental reference for a system of weights and measures , against which all other measuring devices are compared.
The standard score in statistics is a signed measure of variance expressed in terms of standard deviations from the mean.; scientists sought to find the most accurate definition for each fundamental measuring units by preparing a model for the measuring unit characterized by the maximum level of accuracy and stability against the time and the environmental changes , these models are called the ...
In antiquity, systems of measurement were defined locally: the different units might be defined independently according to the length of a king's thumb or the size of his foot, the length of stride, the length of arm, or maybe the weight of water in a keg of specific size, perhaps itself defined in hands and knuckles.
Nonetheless, the residual and irreducible instability of a physical IPK undermined the reliability of the entire metric system to precision measurement from small (atomic) to large (astrophysical) scales. [39] By avoiding the use of an artefact to define units, all issues with the loss, damage, and change of the artefact are avoided. [1]: 125
In the classical definition, which is standard throughout the physical sciences, measurement is the determination or estimation of ratios of quantities. [14] Quantity and measurement are mutually defined: quantitative attributes are those possible to measure, at least in principle.
In the SI, the standard metre is now defined as exactly 1 ⁄ 299 792 458 of the distance that light travels in a second. [ 10 ] [ 11 ] The metre can be realised by measuring the length that a light wave travels in a given time, or equivalently by measuring the wavelength of light of a known frequency.
The definition, agreement, and practical use of units of measurement have played a crucial role in human endeavour from early ages up to the present. A multitude of systems of units used to be very common. Now there is a global standard, the International System of Units (SI), the modern form of the metric system.
SI base units Name Symbol Measure Post-2019 formal definition [1] Historical origin / justification Dimension symbol; second: s time "The second, symbol s, is the SI unit of time.