The problem is that there are two entities out there calling themselves
which is the square root of the sample variance if is
a sample of size from some population. Or the square root of
the variance of a random variable which takes the values
with equal probability.
which is the square root of the bias corrected estimate of the
population variance that is a sample of size from.
Now which you use is up to you and could depend on what you wish to
do with it. But apparently is now commonly used in software
packages as the definition of standard deviation - with poor justification.
However for large samples these are very close and so the difference does
not matter, but if I were you I would stick to .