Section 51.18.010 - Availability-Rules-Coverage period(1) The department shall offer a retrospective rating plan to insure the workers' compensation obligations of employers and groups of employers. The plan is to be made available to any employer or group of employers who: (a) Voluntarily elects to participate in the plan; and(b) Meets the requirements of this chapter and rules adopted by the department under subsection (2) of this section.(2) The retrospective rating plan shall be consistent with recognized insurance principles and shall be administered according to rules adopted by the department. Rules adopted under this section shall encourage broad participation by qualified employers and sponsors of retrospective rating groups.(3) Each retrospective rating group approved by the department under this chapter shall select a coverage period and may be renewed at the end of each coverage period. For the purposes of this section, "coverage period" means a twelve-month period provided by the department by rule.