Deviation Taken from Assumed Mean
This methods is assorted when the arithmetic average is a fractional value. Taking deviation from fractional value would be a very difficult and tedious task. To save time and labor we apply short cut methods deviations are taken from and assumed mean. The followings are the steps.
a.Assume any one of the item in the series as an average(A).
b.Find out the deviation from the assumed mean i e, X-A denoted by d.
c.Find out the total of the deviation i ,e, ∑d.
d.Square the deviations i ,e, d2 and add up the squares of devotion i ,e, ∑d2.
e.Then substitute the values in the followings formula.
= √ ∑d2- / N - ( ∑d2)/N 2
Where d stands for the deviation from assumed mean = ( X-A)
Note : If the actual mean in fraction then it is better to take deviations form an assumed mean for avoiding too much calculation.