The Illinois Department of Revenue requires forms (Real Estate Transfer Declarations) to be filed with the County Recorder's Office when sales of property take place. These forms contain information on the sale and must be signed by the buyers and sellers of the property.
When the sales are determined by the Department of Revenue to be "arm's length" (advertised in the open market with neither party forced to buy or sell), they are compared to the assessment applied by the local assessor from the year prior, resulting in a sales ratio. The Department then adjusts the results of the ratios by adjustments assessors and multipliers have made to prior year assessments.
These adjustments are made because State law defines one-third of market value assessments as being based on one-third of market value for the three prior years.