• Scientific calculators make the computation of standard deviation (σ) easy.
• However, if the number of observations is extremely large even it becomes tedious.
• In such cases, N values are taken as a sample from them and correction applied to the formulae.
• It is called Bessel's correction in which (N – 1) replaces N in the formulae of σ² and σ.
• σ is specifically known as sample standard deviation.

σ² =

and σ =