Arbitrary unit
In science and technology, an arbitrary unit (abbreviated arb. unit, )or procedure defined unit (p.d.u.)is a relative unit of measurement to show the ratio of amount of substance, intensity, or other quantities, to a predetermined reference 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 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 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. (Wikipedia).
