
Question:
What is Standard Deviation ? 

Answer:
The standard deviation
of a set of numbers, such as heights of people is a measure of how
spread out the numbers are from the mean. A natural way to measure
spread is as follows:
 Find how each number deviates from the mean;
 Since the signs of the deviations, positive or negative, are
not of importance (for example 2 above the mean is the same deviation
as 2 below the mean) it is sensible to make the deviations positive
by squaring them
 Find the total of the squared deviations
 Average out this total by dividing it by the number of items (in order to make fair comparison with other sets of data)
 Take the square root to return to the original units (centimeters, rather than square centimeters in the case of heights)
The formula of standard deviation :


