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# pandas.stats.moments.ewmstd

`pandas.stats.moments.``ewmstd`(arg, com=None, span=None, halflife=None, min_periods=0, bias=False, time_rule=None)

  Exponentially-weighted moving std

  Parameters : | arg : Series, DataFrame com : float. optional Center of mass: , span : float, optional Specify decay in terms of span, halflife : float, optional Specify decay in terms of halflife, :math: alpha = 1 - exp(log(0.5) / halflife) min_periods : int, default 0 Number of observations in sample to require (only affects beginning) freq : None or string alias / date offset object, default=None Frequency to conform to before computing statistic time_rule is a legacy alias for freq adjust : boolean, default True Divide by decaying adjustment factor in beginning periods to account for imbalance in relative weightings (viewing EWMA as a moving average) bias : boolean, default False Use a standard estimation bias correction
  Returns : | y : type of input argument

  Notes

  Either center of mass or span must be specified

  EWMA is sometimes specified using a “span” parameter s, we have have that the decay parameter is related to the span as

  where c is the center of mass. Given a span, the associated center of mass is

  So a “20-day EWMA” would have center 9.5.

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