Sales Ratio Study

The three approaches to value are used in single property appraisal, however, considering the magnitude of the assessment jurisdiction, the State of Illinois uses a mass appraisal system. This is a statistical analysis of sales -vs- assessments that encompass a wider geographic area, such as an entire county, township or neighborhood. This analysis is called a Sales Ratio Study; it is used to calculate the ratio of the assessment to the sale. In Illinois this ratio is to be 33.333% of the market value of the property.

The compiled sales ratios from year to year give the State Department of Revenue an indication of the overall market trend as compared to the existing assessments in a jurisdiction. If the sale study ratios are predominately below 33.333% the assessments must be increased; if above 33.333% the assessments must be decreased. The State advises each County of the results of this annual study. In turn, the County compiles sales studies for each Township and advises the Township Assessor of the overall increase or decrease of the Township's assessments.

In DuPage County, the Township Assessors take this one step further. The Assessor, using the same parameters, study individual neighborhoods, subdivisions, condominium complexes, etc., to attain more accurate market trend information for that particular area. This is done so one area's market trend does not impact another and greater uniformity and accuracy is attained.