Sample standard deviation

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 =