Complex sentence: In statistics, the 68--95--99.7 rule--or three-sigma rule, or empirical rule--states that for a normal distribution, nearly all values lie within 3 standard deviations of the mean.
Split into simpler sentences: In statistics, the 68--95--99.7 rule, also known as the three-sigma rule or empirical rule, states that nearly all values lie within 3 standard deviations of the mean in a normal distribution. Nearly all (99.73%) of the values lie within 3 standard deviations of the mean.