ANATOMY OF THE EMPIRICAL RULE (also known as the 68-95-99.7 Rule) The Empirical Rule applies to a specific type of distribution called a bell-shaped distribution (Normal Distribution) as shown below. The Empirical Rule states that for a bell-shaped distribution, approximately: 68.26% of the observations lie within one standard deviation of the mean. 95.44% of the observations lie within two standard deviations of the mean. 99.74% of the observations lie within three standard deviations of the mean. NOTE: For distributions of any shape there is a more general rule, Chebyshev’s theorem. It states that at least 75% of all scores will fall within 2 standard deviations above and below the mean; and at least 89% of all scores will fall within 3 standard deviations of the mean. 68.26% 95.44% 99. 74%