Sample Standard Deviation vs. Population Standard Deviation?

Question

I have an HP 50g graphing calculator and I am using it to calculate the standard deviation of some data. In the statistics calculation there is a type which can have two values: Sample Population I didn't change it, but I kept getting the wrong results for the standard deviation. When I changed it to "Population" type, I started getting correct results! Why is that? As far as I know, there is only one type of standard deviation which is to calculate the root-mean-square of the values! Did I miss something?

Answer 1

The population standard deviation is divided by the population size (N), while the sample standard deviation is divided by the sample size minus 1 (n-1).