Mich. Comp. Laws § 38.461

Sec. 11. Wherever possible, vacancies shall be filled by promotion. Promotion shall be made from among employees, in the grades nearest below the grade in which the vacancy exists, qualified by training and experience to fill the vacancy and whose length of service in such lower grade entitles them to consideration. The commission, for the purpose of promotion, shall rate the qualified employees in the lower grades on the basis of their service record, 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.