Mich. Comp. Laws § 404.204

Sec. 4. Such contract with such institute for the cure of said cases shall be made and entered into with one which is located in the state of Michigan, that can satisfy said board that not less than 75 per cent of the persons having taken a full course of treatment, consisting of not less than 4 weeks, have been cured and have remained cured for at least 1 year thereafter.