68-95-99.7 Rule 0.40 0.35 0.30 68.27% 0.25 Probability Density 0.20 95.45% 0.15 0.10 99.73% 0.05 0.00 M-30 -20 H+0 u+20 u+ 30 68% of the data is within 1 standard deviation, 95% is within 2 standard deviation, 99.7% is within 3 standard deviations Figure 6.5: Normal Distribution Empirical Rule