The standard deviation of a data set represents how much spread a normally distributed set of data has. A smaller standard deviation means that the data is clustered around the mean, while a larger standard deviation means that the data is spread out more widely under the curve. The empirical rule states that 68% of scores will fall within one standard deviation above and below the mean, 95% will fall within 2 standard deviations, and 99.7% will fall within 3 standard deviations.