# Measurement

Determination of the magnitude of a quantity by comparison with a standard for that quantity. Quantities frequently
measured include time, length, area, volume, pressure, mass, force, and energy. To express a measurement, there must
be a basic unit of the quantity involved, e.g., the inch or second, and a standard of measurement (instrument) calibrated
in such units, e.g., a ruler or clock. For convenience, such a standard is usually marked off both in multiples and in
fractions of the basic unit. Although various systems of units exist for measuring different quantities
(see weights and measures), the most important and widely used are the
metric system and the English units of measurement.
Certain units have been defined for special applications, e.g., the light-year and parsec in astronomy and the angstrom in physics.
Measurement is one of the fundamental processes of science. It provides the data on which new theories are based and by which older
theories are tested and retested. A good measurement should be both accurate and precise. Accuracy is determined by the care taken
by the person making the measurement and the condition of the instrument; a worn or broken instrument or one carelessly used may
give an inaccurate result. Precision, on the other hand, is determined by the design of the instrument; the finer the graduations
on the instrument’s scale and the greater the ease with which they can be read, the more precise the measurement. The choice
of the instrument used should be appropriate to the desired precision of the results. The human foot may be a suitable instrument
for pacing off short distances if precision is not important; at the other extreme, the interferometer is used for extremely precise
measurements of distance in science. There is a basic distinction between measurement and counting. The result of counting is exact
because it involves discrete entities that are not subdivided into fractions. Measurement, on the other hand, involves entities that
may be subdivided into smaller and smaller fractions and is thus always an estimate. This distinction between measurement and counting
seems, on the surface, to break down at the atomic level, where the quantum theory reveals that not only mass (in the form of elementary
particles and atoms) but also many other quantities occur only in discrete units, or quanta. It would seem, therefore, that one could,
in theory, reduce measurement to counting at this level. However, the quantum theory also places limitations on the possibility of
counting, stressing such concepts as the wavelike nature and indistinguishability of particles and proposing the uncertainty principle
as an absolute limitation on certain pairs of related measurements.

source: The Columbia Encyclopedia, Sixth Edition. 2001.