The empirical rule states that most of the data will be within three standard deviations in a normal distribution. The 68% of the data will be within one standard deviation, the 95% will be within two standard deviations, and 99.7% of the data will be within three standard deviations.
A normal distribution is a continuous distribution in which values around the mean are the most frequents. It can also be called a bell-shaped distribution.
