35 ILCS 200/17-15
The Department shall forward to the County Clerk of each county in each year its estimate of the percentage, established under Section 17-5, to be added to or deducted from the aggregate of the locally assessed property in that county, other than property assessed under Sections 10-110 through 10-140 and 10-170 through 10-200. The percentage relationship to be certified to each county by the Department as provided by Section 17-25 shall be determined by the ratio between the percentage estimate so made and forwarded, as provided by this Section, and the level of assessments of the assessed valuations as made by the assessors and thereafter finally revised by the board of review of that county. Such estimate shall be forwarded by the Department to the County Clerk of any County within 15 days after the chief county assessment officer files with the Department an abstract of the assessments of the locally assessed property in the county, as finally revised. The abstract shall be in substantially the same form as required of the County Clerk by Sections 9-250 and 9-255 after completion of the revisions thereafter to be made by the board of review of the county, except that the abstract shall specify separately the amount of omitted property, and the amount of improvements upon property assessed for the first time in that year. The chief county assessment officer shall forward the abstract to the Department within 30 days after returning the county assessment books to the county board of review.
35 ILCS 200/17-15