science Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earliest archeological evidence ...

and

technology Technology is the application of knowledge to reach practical goals in a specifiable and reproducible way. The word ''technology'' may also mean the product of such an endeavor. The use of technology is widely prevalent in medicine, scien ...

, an arbitrary unit (abbreviated arb. unit, '' see below'') or procedure defined unit (p.d.u.) is a relative

unit of measurement A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multi ...

to show the ratio of

amount of substance In chemistry, the amount of substance ''n'' in a given sample of matter is defined as the quantity or number of discrete atomic-scale particles in it divided by the Avogadro constant ''N''A. The particles or entities may be molecules, atoms, io ...

, intensity, or other quantities, to a predetermined

reference Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a '' name'' ...

measurement. The reference measurement is typically defined by the local laboratories or dependent on individual measurement apparatus. It is therefore impossible to compare "1 arb. unit" by one measurer and "1000 arb. unit" by another measurer without detailed prior knowledge on how the respective "arbitrary units" were defined; thus, the unit is sometimes called an unknown unitThe unit only serves to compare multiple measurements performed in similar environment, since the ratio between the measurement and the reference is a consistent and

dimensionless quantity A dimensionless quantity (also known as a bare quantity, pure quantity, or scalar quantity as well as quantity of dimension one) is a quantity to which no physical dimension is assigned, with a corresponding SI unit of measurement of one (or 1 ...

independent of what actual units are used. Units of such kind are commonly used in fields such as physiology to indicate substance concentration, and

spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...

to express spectral intensity. When the reference measurement is precisely defined and internationally agreed upon, arbitrary units can also be a unit capable of public comparison. One example of a publicly defined arbitrary unit is the WHO International Unit.

Abbreviations and nomenclature

Abbreviations for "arbitrary unit" include: arb. unit, arb. u., AU, and a.u. Among these, "AU" and "a.u." are common abbreviations for

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...

s and atomic units. For this reason,

Physical Review ''Physical Review'' is a peer-reviewed scientific journal established in 1893 by Edward Nichols. It publishes original research as well as scientific and literature reviews on all aspects of physics. It is published by the American Physical ...

journals, the Japanese Journal of Applied Physics, and an increasing number of other

academic journal An academic journal or scholarly journal is a periodical publication in which scholarship relating to a particular academic discipline is published. Academic journals serve as permanent and transparent forums for the presentation, scrutiny, and ...

s, recommend against using "a.u." (Jpn. J. Appl. Phys recommends "arb. unit" instead). In UCUM, arbitrary unit is represented as " rb'U. While arbitrary unit is not a formally recognized unit,

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

and

IFCC The International Federation of Clinical Chemistry and Laboratory Medicine or IFCC is a global organization that promotes the fields of clinical chemistry and laboratory medicine. It was established in 1952 as the International Association of Clin ...

recognize the need to deal with "unknown units", and decided in 2008 to use the term "procedure defined unit" in their database. The decision forbids using factors or denominators in conjunction with p.d.u. (such as "p.d.u./L").

References

{{DEFAULTSORT:Arbitrary Unit Units of measurement