The empirical rule describes how points are clustered in a normally-distributed data set. It states that 95% of data fall within two standard deviations of the mean.

J. S. Maritz, T. Lwin, Construction of Simple Empirical Bayes Estimators, Journal of the Royal Statistical Society. Series B (Methodological), Vol. 37, No. 3 (1975), pp. 421-425. ... For terms and use ...