SD (Standard Deviation): The standard deviation (σ) is the square root of the mean of the sum of the square of the difference between the value of the mean of those values and of the particular value in connection with the normal distribution. Statistical calculation for expressing the variability or spread of data is SD. The larger the spread of the numbers in a data set, the larger the standard deviation. It is the square root of the variance.

In analytical chemistry one of the most common Statistical terms employed is the Standard Deviation of a population of observation. This is also called the root mean square deviation as it is the square root of the mean of the sum of the squares of the difference between the values and the mean of those values and is of particular value in connection with the normal distribution.

It measures the absolute variability of a distribution or how much your data is spread outaround the mean or average. More precisely, Standard deviation is a measure of the average distance between the valuesof the data in the set and the mean.