The standard deviation is a measure of the variation in a distribution or set of data. The term is used in probability and statistics and remains the most common measure of statistical dispersion. The standard deviation is usually denoted with the letter ?.
The standard deviation is used to summarize continuous data. It can't be applied to categorical data. Normally, it is only appropriate when the data isn't significantly skewed or has outliers. The standard deviation was formulated by Galton in the 1860s. It is defined as the root-mean-square of the values from their mean, or as the square root of the variance.