35 ILCS 200/9-210
The chief county assessment officer in a county with less than 3,000,000 inhabitants shall act as an equalizing authority for each county in which he or she serves. The officer shall examine the assessments in the county and shall equalize the assessments by increasing or reducing the entire assessment of property in the county or any area therein or of any class of property, so that the assessments will be at 33 1/3% of fair cash value. The equalization process and analysis described in this Section shall apply to all property except farm and coal properties assessed under Sections 10-110 through 10-140 and 10-170 through 10-200.
For each township or assessment district in the county, the supervisor of assessments shall annually determine the percentage relationship between the estimated 33 1/3% of the fair cash value of the property and the assessed valuations at which the property is listed for each township, multi-township or assessment district. To make this analysis, he or she shall use property transfers, property appraisals, and other means as he or she deems proper and reasonable.
With the ratio determined for each township or assessment district, the supervisor of assessments shall then determine the percentage to be added to or deducted from the aggregate assessments in each township or assessment district, other than property assessed under Sections 10-110 through 10-140 and 10-170 through 10-200, in order to produce a ratio of assessed value to fair cash value of 33 1/3%. That percentage shall be issued as an equalization factor for each township or assessment district within each county served by the chief county assessment officer. The assessment officer shall then change the assessment of each parcel of property by application of the equalization factor.
35 ILCS 200/9-210