### What is standard deviation and the ’65-95-99.7′ rule?

**Standard deviation** is a measure of how spread out numbers are. The percentage of data that lies within a certain number of deviations is always the same. This is where the ’65-95-99.7′ rule comes in. If a set of data is normally distributed, we know that 68% of the data lies one standard deviation from the mean, 95% lies within the two standard deviations from the mean, and 99.7% lies within the three standard deviations from the mean. The following video introduces normal distribution, standard deviation and the 68-95-99.7 rule in HSC Standard Math.