S.D. Codified Laws § 3-12C-1201
For any generational member, the term, final average compensation, means the highest average annual compensation earned by a member during any period of twenty consecutive calendar quarters during the member's last forty calendar quarters of membership in 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 twenty consecutive calendar quarters.
For any member who has less than twenty 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-1201