The variance measures the variability of observations of a data set from its mean value. The positive square root of the variance is called standard deviation. The coefficient of standard deviation is equal to the ratio of standard deviation to mean. The formula for standard deviation for a grouped data is;

S= [ summation f*x

x= observation

f= frequency of the observation

n= number of observations

S= [ summation f*x

^{2}/n - (summation f*x/n)^{2}]^{1/2}x= observation

f= frequency of the observation

n= number of observations