S.D. Codified Laws § 3-12C-108

For purposes of this chapter, "effective rate of interest" means the interest at an annually compounded rate to be established by the board for each fiscal year. The rate may be no greater than ninety percent of the average ninety-one day United States treasury bill rate for the immediately preceding calendar year and in no event may the rate be more than the rate established by the board pursuant to § 3-12C-227, for investment return for purposes of the actuarial valuation. If a member withdraws contributions pursuant to § 3-12C-602 or 3-12C-604, or if benefits are payable under § 3-12C-409, the interest must be as annually compounded on the preceding June thirtieth.