Empirical rule

The empirical rule, also called the 68-95-99.7 rule, describes how data cluster in a normal distribution. About 68 percent of values fall within 1 standard deviation of the mean, about 95 percent within 2, and about 99.7 percent within 3. For example, if adult heights are normal with mean 68 inches and standard deviation 3 inches, about 95 percent fall between $68 - 2(3) = 62$ and $68 + 2(3) = 74$ inches. It gives quick estimates without needing a z-table.