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Assessor’s Office improved residential property assessment model unveiled for Tax Year 2018 reassessment

Early indicators of accuracy meet industry standards and criteria for minimal regression


Chicago - Cook County Assessor Joseph Berrios has announced that the first reassessment of residential properties using an improved assessment model has met the longstanding goal of reducing regression. This creates more accurate assessed values of homes.

The system for residential property assessment in Cook County has been in existence for 40 years. Assessor Berrios inherited that system and its issues. He and his staff pledged to work with the Civic Consulting Alliance (CCA) and Tyler Technologies to improve residential assessment modeling and related processes. Work with Tyler actually began years ago.

"Improving residential assessment has been a priority during my time in office. It pleases me tremendously to see we are achieving what we set out to do, and new resources now available to us from CCA and Tyler have helped. This is a huge step forward for Cook County," Assessor Berrios said.

The Assessor added, "I thank County Board President Toni Preckwinkle for her help with the Tyler Technologies contract, CCA for its important role, CCA expert Josh Myers, Tyler-affiliated expert Dr. Richard Borst and Tyler for its overall fine work. Everyone in the Assessor’s Office worked diligently to meet the goal of improving the assessment process.”

The Assessor’s Office triennially reassesses one-third of the more than 1.8 million parcels of property located in Cook County. In this tax year (2018), the reassessment is of all properties in the City of Chicago.

Rogers Park Township, one of the eight townships comprising the City, is the first township whose reassessment notices have been mailed. Approximately 11,000 parcels of residential real estate were reassessed there using the now-improved model. Numerous indicators including coefficient of dispersion (COD) are well within industry standards of the International Association of Assessing Officers (IAAO).

This most recent work reflects interactive collaboration between the Assessor’s Office and newly available, renowned valuation and modeling experts and researchers at CCA and Tyler.

Due to the sophistication of these state-of-the art improvements in techniques, the model performance was superior. Results include:

  • Both linear additive and multiplicative (a/k/a log-linear) model structures were evaluated in light of IAAO standards.
  • COD, coefficient of variation (COV), price-related differential (PRD) and price-related bias (PRB) are all well within IAAO standards.
  • The multiplicative model structure was chosen because of its superior performance measures.
  • Outlier identification was based on statistically-founded and defensible methods
  • Geostatistical methods were used to derive a location influence factor intended to improve model performance. The location factor variable was significant and helped to improve accuracy.
  • The valuation procedure used the comparable sales direct market comparison method of valuation.
  • Performance measures of accuracy ultimately obtained were superior to traditional modeling techniques
  • Geospatial analytic methods were employed to ensure that there was no spatial bias in the valuation model. Model performance demonstrated a lack of spatial clustering.

"We have never wavered from our commitment to reducing regression and assessing property as accurately and fairly as possible," said Assessor Berrios.

Reassessment notices contain proposed property values for Tax Year 2018, which will be reflected on Second-Installment Tax Bills to be mailed and due in the summer of 2019. Each "Assessed Value" is 10% of a home’s estimated market value.


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