def _calculate(self, startingPercentage, endPercentage, startDate, endDate):
        """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
        """
        # get the defined subset of error values
        errorValues = self._get_error_values(startingPercentage, endPercentage, startDate, endDate)
        errorValues = filter(lambda item: item is not None, errorValues)

        return sorted(errorValues)[len(errorValues)//2]
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