35 ILCS 200/17-5
The Department shall act as an equalizing authority. It shall examine the abstracts of property assessed for taxation in the counties and in the assessment districts in counties having assessment districts, as returned by the county clerks, and shall equalize the assessments between counties as provided in this Code. Except as hereinafter provided, the Department shall lower or raise the total assessed value of property in each county as returned by the county clerk, other than property assessed under Sections 10-110 through 10-140 and 10-170 through 10-200, so that the property will be assessed at 33 1/3% of its fair cash value.
The Department shall annually determine the percentage relationship, for each county of the State, between the valuations at which locally-assessed property, other than property assessed under the Sections 10-110 through 10-140 and 10-170 through 10-200, as listed by assessors and revised by boards of review, and the estimated 33 1/3% of the fair cash value of the property. To make this analysis, the Department shall use property transfers, property appraisals, and other means as it deems proper and reasonable.
With the ratio determined for each county, the Department shall then determine the percentage to be added to or deducted from the aggregate reviewed assessment on property subject to local assessment jurisdiction, other than property assessed under the Sections cited above, to produce a ratio of assessed value to 33 1/3% of the fair cash value equivalent to 100%.
35 ILCS 200/17-5