In the R Data Analyst “Measure Variability” lesson, we are told that
In the previous screen, we stated that statisticians agree that n-1 is better than n or n-2 for calculating the sample standard deviation. An argument supporting this comes from the fact that the sample variance s^2 which uses n-1 is an unbiased estimator for the population variance σ^2
So my questions are, what exactly is an unbiased estimator? How is just stating that “variance is an unbiased estimator” enough of an argument for the use of n-1? Why is the standard deviation of a sample not an unbiased estimator of of the population SD?