The Practice of Statistics for AP*, 4thEdition Glossary© BFW Publishers 2011Chapter 2The 68-95-99.7 RuleAlso known as the "empirical rule." In the Normal distributionwith meanμand standard deviationσ, (a) Approximately 68% of the observations fallwithinσof the meanμ, (b) Approximately 95% of the observations fall within 2σofμ,and (c) Approximately 99.7% of the observations fall within 3σofμ.Cumulative relative frequency graphUsedto examine location within a distribution.Cumulative relative frequency graphs begin by grouping the observations into equal-width classes. The completed graph shows the accumulating percent of observations asyou move through the classes in increasing order.Density curveA curve that (a) is always on or above the horizontal axis, and (b) hasexactly 1 area underneath it. A density curve describes the overall pattern of adistribution. The area under the curve and above any interval of values on the horizontal
