def modify_average(old_average, old_member_count, new_value, round_value):
    """Modify average value (e.g. mass) for an averaged feature on addition of a new member. i.e. if the average value
    for a feature is 4 based on an average from 4 input values, then adding a new value (6) will make the new average
    (4 * 4 + 6)/5

    """
    modified_value = round(((old_average * old_member_count) + new_value)/(old_member_count + 1), round_value)

    return modified_value