N.D. Cent. Code § 26.1-12-30

In determining the rate of dividend due a given trade, occupation, or profession, if the dividend is allowed, the income received and losses sustained must be tabulated for a period of not less than five years immediately preceding the determination of the dividend rate, and the return dividend to policyholders must be based upon the experience of such period after deduction for expenses and allowances for reserves as required by law.