S.D. Codified Laws § 3-12C-1101
For any foundation member whose contributory service concluded before July 1, 2021, the term, final average compensation, means the highest average annual compensation earned by a member during any period of twelve consecutive calendar quarters during the member's last forty calendar quarters of membership in the system including time during which the member was not a member but for which the member has received credit under the system.
For purposes of determining final average compensation if periods of contributory service are separated by breaks, any service earned from covered employment may be aggregated to constitute a period of twelve consecutive calendar quarters. For any member who has less than twelve but more than four calendar quarters of membership in the system, the member's final average compensation shall be based on the compensation received in all quarters of membership. For any member who has four calendar quarters of membership or less, the member's final average compensation shall be based on the member's annual compensation.
SDCL 3-12C-1101