The Variance and Standard Deviation

The Variance is defined as the sum of squares of the deviations of the observations from the Mean divided by one less than the total number of observations.  The Standard Deviation  () is defined as the positive square root of the Variance.  When the distribution considers the entire Population as a whole, the variance is signified by the Roman numeral Theta squared.

Two other terms that occur less frequently but are helpful in describing the shape of a histogram and- a term you will become intimate with later, a population distribution- are Kurtosis and Skewness.