def calculate_change(current_value, previous_value):
    """
    Calculate change in value given current value and previous value. Can be either +ve or -ve
    :param current_value: str
    :param previous_value: str
    :return: str or na (convert number to string to make data type consistent)
    """
    if not previous_value:
        pass
    else:
        diff = int(current_value) - int(previous_value)
        if int(previous_value):
            return str(diff / int(previous_value))

    return 'na'