What is the Empirical Rule?

The Empirical Rule states that if data follows a normal distribution, approximately 68% of the observations fall within 1 standard deviation of the mean; 95% within 2 standard deviations; and 99.7% within 3 standard deviations. The implications of this rule are useful in outlier detection, as if an observation is further than 3 standard deviations from the mean and the distribution appears to be roughly normal, it is a good indication that there is something usual about that observation. As many types of data naturally follow a normal distribution, this rule provides a convenient mechanism for gauging how likely or unusual an observation is to occur.