Measurement is the estimation of a physical quantity such as distance, energy, temperature or time. Measurement is fundamental in science; it is one of the things that distinguishes it from pseudoscience. It is also essential in industry, commerce, engineering, construction, manufacturing, pharmaceutical production and electronics.
Measurements find the ratio of some physical quantity to a standard quantity of the same type; thus a measurement of length is the ratio of a physical length to some standard length, such as a metre. Measurements are usually given in terms of a real number times a unit of measurement, for example: 2.53 metres.
Two distinctly different kinds of measurement are the marker and the span:
- 3:05 p.m. GMT, 3 June 2001 is a marker
- 15 minutes is a span
Both must be given in reference to some standard, and both involve some necessary error.
A count, as distinct from a measurement, may be exact: I have exactly two ears. On the other hand, large counts such as the number of people in a crowd often use estimation, even when an exact count is theoretically possible. In the natural sciences, the act of measuring often requires an instrument designed and calibrated for that purpose such as a thermometer, speedometer, weighing scale or voltmeter.
In addition to the standard use of the word measurement, the word is often loosely used to mean any number assigned to a physical object. Thus the wind chill factor, which has no scientific validity, or IQ, which is just a count of correct answers on a test, are loosely called measurements. Measurements carried out by surveys are strictly speaking counts rather than measurements, but may be converted to measurements by the use of statistics.