Describe what the 1-2-3 rule is.
The 1-2-3 rule says that regardless of the exact shape of the normal distribution, 68.26% of all observations are within +1 and -1 s.d. of the mean, and 95.44% of all observations are within +2 and -2 standard deviations of the mean and 99.72% of the observations are within +3 and -3 standard deviations of the mean.