If your data represents only a sample of the entire population for the variable, then use the STDDEV_SAMP function. For information, see STDDEV_SAMP.

As the sample size increases, the values for STDDEV_SAMP and STDDEV_POP approach the same number, but you should always use the more conservative STDDEV_SAMP calculation unless you are absolutely certain that your data constitutes the entire population for the variable.