Ask the Assessor - Ensuring Equity in Asessments
Reprinted by permission of Kane County Bar Association from Mark D. Armstrong, “Ask the Assessor," Bar Briefs, September/October 2025, pp. 34-35. © 2025 by Kane County Bar Association.
Ask the Assessor!
By Mark D. Armstrong, CIAO-M
Question: Over the past several years, I've been reading about bias in the appraisal profession. What steps do assessors take to ensure equity in assessments?
Answer: In Illinois, equity in property taxation has been enshrined in our constitution since our admission to statehood in 1818: “[t]hat the mode of levying a tax shall be by valuation, so that every person shall pay a tax in proportion to the value of the property he or she has in his or her possession."[i] In other words, the more valuable one's property, the greater proportion of the tax burden one bears. This provision has continued through all of our constitutions; the current constitution provides that “taxes upon real property shall be levied uniformly by valuation ascertained as the General Assembly shall provide by law."[ii]
It is important to note that our constitution does not require absolute equity. The Illinois Supreme Court addressed this topic in 1960, ruling that “[t]he constitutional provision for uniformity does not require that property be assessed on any particular day or on the same day; nor does it call for a mathematical equality. The requirement is satisfied if the intent is evident to adjust the burden with a reasonable degree of uniformity and if such is the effect of the statute in its general operation. A practical uniformity, rather than an absolute one, is the test."[iii] But if a practical uniformity, rather than absolute uniformity, is the test, how is assessment equity measured?
Use of Sales-Ratio Studies to Measure Assessment Equity
By statute, the Illinois Department of Revenue prepares sales-ratio studies on an annual basis to measure assessment equity. These studies begin by comparing the selling prices of real property to the assessed values placed upon them.[iv] For example, a property with a sale price of $385,000 and an assessed value of $122,361 would have a sales ratio of ($124,588 ÷ $385,000) 32.36%. This calculation is repeated for all valid sales in a given year, and the median of these ratios becomes the median ratio for that year. Invalid sales are excluded, such as sales that are not arm's length transactions.[v] However, some arm's length sales are excluded. A common example is the sale of farmland, because farmland assessments are not based on market value; instead, farmland is assessed based on its use value (ability to generate income from farming based on the soil's ability to produce a crop).[vi]
By Illinois law, the average of the median ratios for the prior three years is used to measure assessment accuracy and develop a countywide equalization factor.[vii] If the three-year average of the median ratios is within 1% of the statutory target of 33.33%. then a 1.0000 equalization factor is issued and no further changes are made.[viii] If there is a variance of greater than 1% then an equalization factor is used to bring the three-year average of the median ratios to 33.33%.[ix] I am pleased to report that Kane County's equalization factor has been 1.0000 every year for since 1987.
But the results of the study are also used to develop three key measures of assessment equity:
- Coefficient of Dispersion (the “COD"), which is “the average deviation of all assessments from the median level."[x] To determine a COD, one must first determine the median level of assessments for the dataset, and then the absolute deviation of each individual ratio from the median level of assessments. The average of those absolute deviations is the COD, and it is expressed as a percentage.[xi] Generally, one will expect larger jurisdictions to have a lower COD than smaller jurisdictions due to the greater number of observations. In Illinois, the target levels for COD are to be no more than 15% for counties with populations of more than 50,000 persons, and no more than 30% for populations of 50,000 or fewer persons.[xii] The International Association of Assessing Officers notes that low CODs (15% or less) are associated with good assessment uniformity.[xiii]
- Coefficient of Concentration (the “COC"), which is “the percentage of ratios falling within a given percentage of the median."[xiv] The Illinois Department of Revenue uses 10% for this measurement.[xv] This measurement often approximates the inverse of COD, but is not as impacted by outliers. While there is no set standard for this measurement in Illinois, higher COCs are preferable to lower COCs.
- Price-Related Differential (the “PRD"), which is a statistic for measuring assessment regressivity or assessment progressivity.[xvi] This is, perhaps, the greatest indication of bias between different levels of home values, and thus is considered by many to be a key measure of assessment equity. Assessments are considered regressive if high-value properties are underassessed relative to low-value properties, and progressive if high-value properties are relatively over-assessed.[xvii] A regressive pattern of assessment will favor wealthier homeowners, and a progressive pattern of assessment will favor poorer homeowners. The PRD is calculated by determining the mean (not the median) assessment ratio in a jurisdiction, then dividing by the aggregate ratio (that is, the sum of all assessments in the study) by the sum of the sale prices. For PRD, a ratio closer to 1.00 is better than a ratio that is further away, irrespective of whether it is higher or lower. In Illinois, the target ratio is between 0.98 and 1.03.[xviii] A ratio higher than 1.03 suggests that assessments are regressive, while a ratio of less than 0.98 suggests that assessments are progressive.[xix]
Application to Assessments in Kane County
So, to paraphrase former New York City Mayor Ed Koch, “How're we doin'?" The best indication of whether assessment equity exists in Kane County will come from the Illinois Department of Revenue's Sales Ratio and Equalization tables, which use a uniform measurement statewide, and are free from any local irregularities that might occur if each county conducted its own study. Each year's sales-ratio study is published as soon as the data for all counties in that year is complete.[xx] The most recent year with data available for all Illinois counties is 2023. Figure 1 shows the data for Kane County and the rest of the Chicago metropolitan area, and it includes a ranking of each county relative to the other 101 counties statewide. To this data, I have added a “statewide overall rank", which reflects all three measures of assessment equity, with each one given an equal weight. 
This data shows Kane County has the second-best coefficient of dispersion in the state; this COD means that the average absolute deviation from the median level of assessments is 11.74%, which is well below both statutory and industry targets of no more than 15%. In Illinois, only Kendall County, with a coefficient of dispersion of 9.59%, has a has a better COD than Kane County.
This data also shows Kane County has the second-best coefficient of concentration in the state; this COC shows that the 57.42% of all assessment ratios are with 10% of the median level of assessments. Again, in Illinois only Kendall County, with a coefficient of concentration of 68.93%, has a has a better COC than Kane County.
Kane County does have the best price-related differential in the state; at 1.00, there is not even the slightest indication of bias or inequity by this measure. Kane County shares this distinction with Boone, Cook, DuPage, and LaSalle Counties. (For the curious, Kendall County has a highly respectable PRD of 1.01, which is tied for sixth best in the state.)
Finally, when the statewide ranking of all three measures of assessment equity is combined, Kane County has the distinction of having the most equitable assessments in Illinois. Given our larger cities with many older properties that lack homogeneity, this is quite an accomplishment! But credit must be given where credit is due: all assessment officers in Kane County, especially our 16 independently elected Township assessors, had a role in making this county the most equitably assessed county in the state.
Your thoughts and comments are always welcome.
* * * *
Mark D. Armstrong, CIAO-M, has 40 years of experience valuing real property and is in his fifth term as Kane County Supervisor of Assessments. He is the President of the County Assessment Officers Association of Illinois, and has drafted part or all of 15 bills that have been made part of the Illinois Property Tax Code. Questions for Ask the Assessor can be sent to Mark at ArmstrongMark@KaneCountyIL.gov.
[i] Ill. Const. 1818, art. VIII, § 20, emphasis added.
[ii] Ill. Const. 1970, art. IX, § 4, emphasis added.
[iii] Apex Motor Fuel Co. v. Barrett, 20 Ill. 2d 395, 401 (Ill. 1960).
[iv] 35 ILCS 200/17-10.
[v] Illinois Department of Revenue Publication 136: Property Assessment and Equalization, April 2016, p. 7.
[vi] Id.
[vii] 35 ILCS 200/17-30.
[viii] 35 ILCS 200/17-25.
[ix] Id.
[x] 35 ILCS 200/4-20.
[xi] Property Assessment Valuation, Second Edition (Chicago: International Association of Assessing Officers, 1996), p. 323.
[xii] 35 ILCS 200/4-20.
[xiii] Property Assessment Valuation at 324.
[xiv] Property Appraisal and Assessment Administration, (Chicago: International Association of Assessing Officers, 1990), p. 532.
[xv] Illinois Department of Revenue Publication 136, p. 4.
[xvi] Property Assessment Valuation, p. 324.
[xvii] Property Appraisal and Assessment Administration, p. 539.
[xviii] Illinois Department of Revenue Publication 136, p. 11.
[xix] Property Assessment Valuation, p. 331.
[xx] A wealth of property tax data can be viewed on the department's web site at https://tax.illinois.gov/research/taxstats/propertytaxstatistics.html; the specific study referenced can be found at https://tax.illinois.gov/content/dam/soi/en/web/tax/research/taxstats/propertytaxstatistics/documents/y2023tbl1.pdf.
