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 d².

f.Substitute  the values  in the followings  formula to get the  standard deviation.

g.Square  the deviation and get d².