Mich. Comp. Laws § 38.415
Whenever possible, vacancies shall be filled by promotion. Promotion shall be made from among employees qualified by training and experience to fill the vacancy, and whose length of service entitles them to consideration. The commission shall, for the purpose of promotion, rate such employees so qualified on the basis of their service record if maintained, experience in the work involved in the vacant position, training and qualification for such work, seniority and war service ratings. Seniority shall be controlling only when other factors are equal. Only 1 name, the highest on the list of ratings, shall be certified. The appointing authority shall then appoint the person so qualified forthwith, or elect to make an original appointment, in which event the procedure for original appointments hereinbefore provided shall be followed.
MCL 38.415