Question: What is Standard Deviation ?


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 :