As discussed in the previous tip, calculating standard deviation is an easy way to determine whether the change you see in absolute numbers is statistically significant, if your data falls outside of two standard deviations of the mean.
In PPC, we tend to focus a lot of attention not only on averages, but on standard deviations from the average. For example, in 2002 the CDC released data about the average height and weight of American adults.
From the central limit theorem we also know that the chances of the actual value falling within three standard deviations of your observed mean are very high (99.7 percent), and that the width of the normal distribution will continue to narrow...
This corresponds to a statistical Z-score of 2, meaning that the number must fall within two standard deviations of the observed mean. Many landing page testers are surprised when the results of their initial test don't hold up if the test is rerun...
