The secretary of revenue, in making such equalization, may add to or deduct from the aggregate assessed valuation of lands, municipal lots, and structures thereon. However, in all cases of addition to or deduction from the assessed valuation of any class of property in the several counties or throughout the state, such additions to or deductions from the assessed valuation shall be based upon competent and tangible evidence and the rate percent of addition or deduction may not be fractional.
SDCL 10-11-50