stddev bug in util.py

[Ted Nienstedt]

Gentlemen, et al:
 
The stddev function in the current, as of 11/03, utils.py module has a
bug.  Corrected code excerpt below, in red; comments in blue.
 
Ted Nienstedt
tnienstedt at telesyn.com
 
 
def mean(values):
    """Return the arithmetic average of the values."""
    return sum(values) / float(len(values))
 
def stddev(values, meanval=None):
    """The standard deviation of a set of values.
    Pass in the mean if you already know it."""
    if meanval == None: meanval = mean(values)
    #return math.sqrt( sum([(x - meanval)**2 for x in values]))
    # the above was missing the crucial divide by n-1 !
    return math.sqrt( sum([(x - meanval)**2 for x in values]) /
(len(values)-1) )
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