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On Wed, Dec 17, 2008 at 9:03 PM, Robert Kern <robert.kern@gmail.com> wrote:
On Wed, Dec 17, 2008 at 19:53, <josef.pktd@gmail.com> wrote:
On Wed, Dec 17, 2008 at 7:58 PM, Tim Michelsen <timmichelsen@gmx-topmail.de> wrote:
Hello, I observed that there are 2 standard deviation functions in the scipy/numpy modules:
Numpy: http://docs.scipy.org/doc/numpy/reference/generated/numpy.std.html#numpy.std
Scipy: http://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.std.html#sci...
What is the difference? There is no formula included within the docstrings.
I suppose that np.std() is for the whole population and scipy.std is designed for a smaller sample in the population. Is that true?
difference between population (numpy) and sample (scipy.stats) variance and standard deviation is whether the the estimator is biased, i.e. 1/n, or not, i.e. 1/(n-1).
It's a shame that the "biased/unbiased" terminology still survives in the numpy.std() docstring. It's really quite wrong.
I find talking about biased versus unbiased estimator much clearer than the population - sample distinction, and degrees of freedom might be more descriptive but its meaning, I guess, relies on knowing about the (asymptotic) distribution of the estimator, which I always forget and have to look up. Josef