code: def _calculate(self, startingPercentage, endPercentage, startDate, endDate):
          
          # get the defined subset of error values
          errorValues = self._get_error_values(startingPercentage, endPercentage, startDate, endDate)
  
          # get the historic mean
          if startDate is not None:
              possibleDates = filter(lambda date: date >= startDate, self._errorDates)
  
              # This piece of code is not required, because _get_error_values already ensured that the startDate
              # was correct. Otherwise it would have thrown an exception.
              #if 0 == len(possibleDates):
              #    raise ValueError("%s does not represent a valid startDate." % startDate)
  
              meanIdx = self._errorDates.index(min(possibleDates))
          else:
              meanIdx = int((startingPercentage * len(self._errorValues)) / 100.0)
  
          mad = sum(errorValues) / float(len(errorValues))
          historicMean = self._historicMeans[meanIdx]
  
          return mad / historicMean
docs: This is the error calculation function that gets called by :py:meth:`BaseErrorMeasure.get_error`.
  
          Both parameters will be correct at this time.
  
          :param float startingPercentage: Defines the start of the interval. This has to be a value in [0.0, 100.0].
              It represents the value, where the error calculation should be started.
              25.0 for example means that the first 25% of all calculated errors will be ignored.
          :param float endPercentage:    Defines the end of the interval. This has to be a value in [0.0, 100.0].
              It represents the value, after which all error values will be ignored. 90.0 for example means that
              the last 10% of all local errors will be ignored.
          :param float startDate: Epoch representing the start date used for error calculation.
          :param float endDate: Epoch representing the end date used in the error calculation.
  
          :return:    Returns a float representing the error.
          :rtype: float