Figure 4.

Areas under the Normal curve. Because the Normal distribution is defined entirely by its mean and standard deviation (SD), the following rules apply: (a) 68.3% of the distribution falls within 1 SD of the mean (i.e. between mean- SD and mean+ SD); (b) 95.4% of the distribution falls between mean-2 SD and mean+2 SD; (c) 99.7% of the distribution falls between mean-3 SD and mean+3 SD; and (d) 95% of the distribution falls between mean-1.96 SD and mean+1.96 SD.