35 ILCS 200/17-25
The assessments of all property, other than property assessed under Sections 10-110 through 10-140 and 10-170 through 10-200, as returned by the county clerks, shall be equalized by adding to the aggregate assessed value thereof in every county in which the Department finds the valuation to be less than 33 1/3% of the fair cash value of the property, the rate per cent which will raise the aggregate assessed valuation to 33 1/3% of fair cash value, and by deducting from the aggregate assessed value thereof, in every county in which the Department finds the valuation to be more than 33 1/3% of the fair cash value, the rate per cent which will reduce the aggregate assessed valuation to 33 1/3% of fair cash value.
However, no equalization factor shall be certified by the Department to raise or reduce the aggregate assessed value of any county in which the aggregate assessed value of property other than that assessed under the Sections cited above, is more than 99% and less than 101% of 33 1/3% of fair cash value.
35 ILCS 200/17-25