Standard deviation is a measure of absolute variation used to calculate the amount of divergence which exists from an expected value and is denoted by the Greek symbol for sigma. Out of all the measures of dispersion and spread, standard deviation is the most frequently utilized and it is advantageous in that it can easily detect for skewness (1).

The variables in this equation are:

σ = standard deviation (in this equation it is denoted by the letter s)

x = each value in the population

xbar = mean of sample

n = number of scores

Reference:

1. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3198538/