Approximately 68% of the data is within one standard deviation of the mean.Approximately 95% of the data is within two standard deviations of the mean.More than 99% of the data is within three standard deviations of the mean.This is known as the Empirical Rule.It is important to note that this rule only applies when the shape of the distribution of the data is bell-shaped and symmetric. We will learn more about this when studying the Normal or Gaussian probability distribution in later chapters.