Variance and Standard Deviation The most important measures of “spread” or dispersion
Sample Variance The variance is the average squared deviation away from the mean. For a sample of size n, divide by n – 1. For a population, divide by n. Sample mean x
The standard deviation The standard deviation is simply the square root of the variance. The variance must always be calculated before the standard deviation.
Population mean μ σ and σ2 are pronounced “sigma” and “sigma squared”, respectively.
The coefficient of variation (CV) The coefficient of variation standardizes the standard deviation by dividing by the mean.
The Empirical Rule * This is actually a normal, continuous distribution, also known as a “bell curve”.
mode = median = mean mode < median < mean mode > median > mean
The amount of skewness is measured using Pearson’s coefficient of skewness (sk):