S.D. Codified Laws § 58-15-15.8

The maximum rate of interest for each policy shall be determined at regular intervals at least once every twelve months, but not more frequently than once in any three-month period. At the intervals specified in the policy, the rate being charged may be increased whenever such increase as determined under an adjustable maximum interest rate would increase that rate by one-half percent or more per year. The rate being charged must be reduced whenever such reduction as determined under § 58-15-15.7 would decrease that rate by one-half percent or more per year.