The Service shall periodically evaluate each employment position. The evaluation may: Document differences among positions; Establish a relative value for the position; and Compare the compensation level of the employment position with the compensation level of a similar employment position in a similar labor market. A job evaluation shall include analysis of the following criteria: Organizational impact; Supervision exercised; Internal and external contacts; Working conditions; and Work requirements. The Service may use a position evaluation to assign a salary grade to an employment position. If the Service reduces the salary grade of an employment position to a lower grade in the salary structure, the Service shall provide an incumbent employee in the employment position with a written explanation for the reduction.