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Measurement is the estimation of a physical quantity such as distance, energy, temperature, or time. 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 standard meter. Measurements are usually given in terms of a real number times a unit of measurement, for example 2.53 meters, but sometimes measurements use complex numbers, as in measurements of electrical impedance. Measurements always involve some error, and so in science measurements are often accompanied by error bounds, as in 2.53 meters plus or minus .01 meters. The study of measurement is called metrology. An old fashioned word for measurement is mensuration.