The standard deviation is a measure of variation that you will come across a lot in studies, journals, and statistics class. It is a basic and fundamental concept in statistics that must be understood in order to understand most other statistics concepts or procedures. By definition, the standard deviation is a measure of variation for interval-ratio variables. It describes how much variation or diversity there is in a distribution. The standard deviation increases or decreases based on how closely the scores cluster around the mean.
The standard deviation is a measure of how spread out the numbers in a distribution are. It indicates how much, on average, each of the values in the distribution deviates from the mean, or center, of the distribution. It is calculated by taking the square root of the variance. Read more about the standard deviation and see an example, plus learn how it is calculated.