In the Standard Normal Distribution of the "Bell-Shaped Curve," 67

In the standard normal distribution of the "bell-shaped curve," 67 percent of all observations fall within one standard deviation about the mean. At two standard deviations, 95 percent of all observations would be included; and at three standard deviations, 99 percent. Carrying this logic out further to six standard deviations, which is what "six sigma" essentially is, would include 99.99966 percent of all observations, equivalent to only 3.4 observations out of a total of one million.