Question

Given a data​ set, how do you know whether to calculate σ or​ s?

Answer 1

`\sigma` is the population standard deviation while s is the sample standard deviation.

When given a data set, one would have to determine if it represented the population or if it was a sample taken from the population.

If the data are a population, then
`\sigma` is calculated. If the data are a sample, then s is calculated.