(Reference) "The empirical rule, also referred to as the three-sigma rule or 68-95-99.7 rule, is a statistical rule which states that for a normal distribution, almost all observed data will fall within three standard deviations of the mean or average. In particular, the empirical rule predicts that 68% of observations falls within the first standard deviation, 95% within the first two standard deviations, and 99.7% within the first three standard deviations."