S.D. Codified Laws § 62-4-25
As to an employee in an employment in which it is the custom to operate throughout the working days of the year, but who is not covered by § 62-4-24, the average weekly wages shall, where feasible, be ascertained by computing the total of the employee's earnings during the period the employee worked immediately preceding the employee's injury at the same grade of employment for the employer by whom the employee was employed at the time of the employee's injury, and dividing such total by the number of weeks and fractions thereof that the employee actually worked. However, if such method of computation produces a result that is manifestly unfair and inequitable or if by reason of the shortness of time during which the employee has been in such employment, or the casual nature or terms of the employment, it is impracticable to use such method, then regard shall be had to the average weekly amount which during fifty-two weeks previous to the injury was being earned by a person in the same grade, employed at the same work, by the same employer, or if there is no person so employed, by a person in the same grade, employed in the same class of employment in the same general locality.
SDCL 62-4-25