figure A-6 Empirical rule According to the empirical rule, roughly 68% of scores in a normal distribution tend to fall within 1 standard deviation of the mean; roughly 95% tend to fall within 2 standard deviations of the mean; and roughly 99% tend to fall within 3 standard deviations of the mean. In a normal distribution in which the mean is 5 and the standard deviation is 1, 68% of scores will fall between 4 and 6; 95% of scores will fall between 3 and 7; and 99% of scores will fall between 2 and 8.