35 ILCS 200/18-230
Only when a new rate or a rate increase or decrease has been approved by referendum held prior to March 22, 2006, the aggregate extension base, as adjusted in Section 18-215 , shall be multiplied by a rate increase (or decrease) factor. The numerator of the rate increase (or decrease) factor is the total combined rate for the funds that made up the aggregate extension for the taxing district for the prior year plus the rate increase approved or minus the rate decrease approved. The denominator of the rate increase or decrease factor is the total combined rate for the funds that made up the aggregate extension for the prior year. For those taxing districts for which a new rate or a rate increase has been approved by referendum held after December 31, 1988 and prior to March 22, 2006, and that did not increase their rate to the new maximum rate for that fund, the rate increase factor shall be adjusted for 4 levy years after the year of the referendum (unless the governing body of a taxing district to which this Law applied before the 1995 levy year that approved a tax rate increase at a general election held after 2002 directs the county clerk or clerks by resolution to make such adjustment for a lesser number of years) by a factor the numerator of which is the portion of the new or increased rate for which taxes were not extended plus the aggregate rate in effect for the levy year prior to the levy year in which the referendum was passed and the denominator of which is the aggregate rate in effect for the levy year prior to the levy year in which the referendum was passed.
35 ILCS 200/18-230