# Measurement

**Measurement** is the act of quantifying a property of an object or relation. Quantification requires there exist a **standard unit**; measurements are expressed as multiples of the standard unit (or as a ratio of the dimension measured to the dimension of the standard unit). A **count** is distinct from a measurement, in that a count quantifies the number of distinct objects, and can thus be exact, while a measurement is never entirely exact.

## Units of measurement

Historically, there have existed a wide variety of standard units; most measurement for scientific and commercial purposes today uses the standard units of the International System of Units (abbreviated **SI** from its name in French, * Système international d'unités*). However, in the United States, much measurement is performed using United States customary units instead.

## Accuracy and precision

**Accuracy** is the measure of correctness of a measurement; **precision** is the measure of the repeatability of a measurement. Systematic errors in the measuring equipment or technique can cause measurements to be inaccurate - to produce a result which is incorrect, and repeatably so. (A simple example would be using a ruler which was not properly calibrated.) Random errors in measuring equipment or technique will cause a lack of precision in a measurement; a measurement is considered more precise when the deviations between repeated measurements of the same quantity are lower.

## References

http://honolulu.hawaii.edu/distance/sci122/SciLab/L5/measure.html