Coefficient of variation A measure of relative variability that expresses standard deviation as a percentage of the mean.
Empirical rule For a specific bell-shaped distribotion ,about 68% of the observations fall in the interval (μ-σ)to(μ+σ) ,about 95% fall in the interval (μ-2σ)to
(μ+2σ),and about 99.7% fall in the interval(μ-3σ)to(μ+3σ).