docs: def std_conversion(period_std, base_period):
      """
      one_period_len standard deviation (or standard error) approximation
  
      Parameters
      ----------
      period_std: pd.DataFrame
          DataFrame containing standard deviation or standard error values
          with column headings representing the return period.
      base_period: string
          The base period length used in the conversion
          It must follow pandas.Timedelta constructor format (e.g. '1 days',
          '1D', '30m', '3h', '1D1h', etc)
  
      Returns
      -------
      pd.DataFrame
          DataFrame in same format as input but with one-period
          standard deviation/error values.
      """
      period_len = period_std.name
      conversion_factor = (pd.Timedelta(period_len) /
                           pd.Timedelta(base_period))
      return period_std / np.sqrt(conversion_factor)