S.D. Codified Laws § 58-20-17
All unallocated compensation loss expense payments made in a given calendar year subsequent to the first three years in which an insurer has been issuing compensation policies shall be distributed as follows: forty percent shall be charged to the policies written in that year, forty-five percent to the policies written in the preceding year, ten percent to the policies written in the second year preceding and five percent to the policies written in the third year preceding, and such payments made in each of the first three calendar years in which an insurer issues compensation policies shall be distributed as follows: in the first calendar year one hundred percent shall be charged to the policies written in that year; in the second calendar year fifty percent shall be charged to the policies written in that year and fifty percent to the policies written in the preceding year; in the third calendar year forty-five percent shall be charged to the policies written in that year, forty-five percent to the policies written in the preceding year and ten percent to the policies written in the second year preceding; and a schedule showing such distribution shall be included in the annual statement.
SDCL 58-20-17