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SD, variance and mean.
A standard deviation calculator measures how spread out a set of numbers is around their mean. A small standard deviation means the values cluster tightly; a large one means they are widely dispersed.
It is a cornerstone of statistics, used in finance (risk), quality control, science and grading to quantify variability, and it also reports the variance, mean and count.
Use the population formula when your data is the entire group; use the sample formula (dividing by n − 1) when it is a sample drawn from a larger population.
Population divides by N and is used when you have every member of the group. Sample divides by n − 1 and is used when your data is a subset that estimates a larger population.
Standard deviation is the square root of the variance. Variance is easier to work with algebraically, but standard deviation is in the same units as the data, making it easier to interpret.
Every value is identical to the mean — there is no spread at all in the data.