[Numpy-discussion] non-standard standard deviation
Dr. Phillip M. Feldman
pfeldman at verizon.net
Sun Nov 29 17:13:11 EST 2009
All of the statistical packages that I am currently using and have used in
the past (Matlab, Minitab, R, S-plus) calculate standard deviation using the
sqrt(1/(n-1)) normalization, which gives a result that is unbiased when
sampling from a normally-distributed population. NumPy uses the sqrt(1/n)
normalization. I'm currently using the following code to calculate standard
deviations, but would much prefer if this could be fixed in NumPy itself:
def mystd(x=numpy.array([]), axis=None):
"""This function calculates the standard deviation of the input using the
definition of standard deviation that gives an unbiased result for
samples
from a normally-distributed population."""
xd= x - x.mean(axis=axis)
return sqrt( (xd*xd).sum(axis=axis) / (numpy.size(x,axis=axis)-1.0) )
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