Measurement (from Old French, mesurement) is the assignment of numbers to objects or events. It is a cornerstone of most natural sciences, technology, economics, andquantitative research in other social sciences.
Any measurement can be judged by the following meta-measurement criteria values: level of measurement (which includes magnitude), dimensions (units), and uncertainty. They enable comparisons to be done between different measurements and reduce confusion. Even in cases of clear qualitative similarity or difference, increased precision through quantitative measurement is often preferred in order to aid in replication.
Measurements are most commonly made in the SI system, which contains seven fundamental units: kilogram, metre, candela, second, ampere, kelvin, and mole. Six of these units are artifact-free (defined without reference to a particular physical object which serves as a standard); the definition of one remaining unit, the kilogram is still embodied in an artifact which rests at the BIPM outside Paris. Eventually, it is hoped that new SI definitions will be uniformly artifact-free.
| Base quantity | Base unit | Symbol | Current SI constants | |
|---|---|---|---|---|
| time | second | s | hyperfine splitting in Cesium-133 | |
| length | metre | m | speed of light in vacuum, c | |
| mass | kilogram | kg | mass of International Prototype Kilogram (IPK) | |
| electric current | Ampere | A | permeability of free space, permittivity of free space | |
| temperature | Kelvin | K | triple point of water, absolute zero | |
| amount of substance | mole | mol | molar mass of Carbon-12 | |
| luminous intensity | candela | cd | luminous efficacy of a 540 THz source |
Source: http://en.wikipedia.org/wiki/Measurement
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