http://www.ck12.org Chapter 5. Normal Distribution
Inflection Points on a Normal Density Curve
We already know from the empirical rule, that approximately 2/3 of the data in a normal distribution lies within 1
standard deviation of the mean. In a density curve, this means that about 68% of the total area under the curve is
withinz−scores of±1. Look at the following three density curves: