L'écart-type ou écart quadratique moyen ou déviation standard, Standard deviation
Ce paramètre de dispersion, est la racine carré de la variance (moyenne arithmétique des carrés des écarts par rapport à la moyenne).
L'écart-type est exprimé dans la même unité que la variable x.
Ainsi, si la variable x est exprimée en centimètres, l'écart-type s'exprime en centimètres.
Anglais : Standard deviation.
The term STANDARD DEVIATION was introduced by Karl Pearson (1857-1936) in 1893, "although the idea was by then nearly a century old" (Abbott; Stigler, page 328). According to the DSB:
The term "standard deviation" was introduced in a lecture of 31 January, 1893, as a convenient substitute for the cumbersome "root mean square error" and the older expressions "error of mean square" and "mean error."
The OED2 shows a use of standard deviation in 1894 by Pearson in "Contributions to the Mathematical Theory of Evolution, Philosophical Transactions of the Royal Society of London, Ser. A. 185, 71-110: "Then will be termed its standard-deviation (error of mean square)."