Calculation of Standard Deviation - Continuous Series: In the continuous series the methods of calculation standard deviation is almost the same as in a discrete frequency distribution. But in a continuous series, mid values of the class intervals are to be found out. The step deviation methods is widely used. Followings are the steps:
a.Find out the mid value of each group or class.
b.Assume one of the mid values as an average and denote it by A.
c. Find out deviation of each mind value from the assumed average a and denote these deviations by d.
d. If the class intervals are equal then take a common factor. Divide each deviation by the common factor and denote this column by
e.Multiply these deviations d by the respective frequencies and get ∑ f d^{2}.
f.Substitute the values in the followings formula to get the standard deviation.
g.Square the deviation and get d^{2}.