When you collect data from a sample, the sample standard deviation is used to make estimates or inferences about the population standard deviation. The population standard deviation formula looks like this: Formula When you have collected data from every member of the population that you’re interested in, you can get an exact value for population standard deviation. Standard deviation formulas for populations and samplesĭifferent formulas are used for calculating standard deviations depending on whether you have data from a whole population or a sample. The empirical rule is a quick way to get an overview of your data and check for any outliers or extreme values that don’t follow this pattern.įor non-normal distributions, the standard deviation is a less reliable measure of variability and should be used in combination with other measures like the range or interquartile range.