The areas within one and two SD's of the mean, respectively, are shaded. The area under the normal curve within one SD of the mean will always be .6827, or approximately 68%. The area within two SD's of the mean will always be .9545, or approximately 95%.
All the plots in Figure 4.5 contain a second horizontal axis, labeled Z . Note that the Z=1 whenever X is one SD above the mean. Similarly, Z=-1 whenever X is one SD below the mean .
If we replace the original X-axis by the Z-axis, then all normal curves will look like Figure 4.6. This is the Z-curve, also called the standard normal curve .
The standard normal curve (or Z-curve) has the following properties:
The empirical rule for the SD is really a special case of the normal curve applied to data. We restate the empirical rule here
Exercise: Revisit Figure 4.2, and examine how closely the data satisfies the empirical rule.