When should I use the sample formula for variance and standard deviation and when should I use the population formula for these measures of variability?

Answer 1

The sample formulas should be used when the observations are a subset of the population of interest. A correction is applied to correct for the sample estimates potential underestimation of the population parameter. When the observations are considered to be the entire population, the population formulas should be used. In situations where sample size is large the difference between the two types of formulas may be negligible; in smaller samples the differences between population and sample formulas will likely be more pronounced.