What is an assessment equity study and how do you request one?

Last updated 2026-07-11

What exactly is an assessment equity study?

An assessment equity study compares assessed values to actual market sale prices across a group of properties in the same jurisdiction. The core metric is the assessment ratio: assessed value divided by sale price. If your county's legal assessment ratio is 100%, your home sold for $400,000, and it's assessed at $440,000, your ratio is 110%. That gap is the problem.

The study doesn't look at just your property. It looks at hundreds or thousands of recent sales in the same area, calculates each property's ratio, and produces summary statistics: the median ratio, the coefficient of dispersion (COD), and sometimes the price-related differential (PRD). Those three numbers tell you two things. Whether the average assessment is close to the legal target. And whether the variation around that average is fair.

The International Association of Assessing Officers (IAAO) sets the professional standards for these studies. Its Standard on Ratio Studies states that "the primary measures used to evaluate assessment performance are the median assessment ratio, the coefficient of dispersion, the price-related differential, and the price-related bias index" ^[1]. That's more than academic jargon. Those are the exact statistics your state oversight agency checks, and the ones you can cite in a formal appeal.

Think of the study as a report card on the assessor. A well-run office wants a median ratio near 100% and a COD below 15% for residential properties, which is the IAAO benchmark. When those numbers drift, some homeowners are paying more than their fair share, and an equity study is how you prove it.

How is an assessment equity study different from a regular appraisal?

A private appraisal tells you what your specific property is worth on a given date. An equity study tells you whether the assessor used the same measuring stick on your property that it used on everyone else. These are two separate legal arguments at a hearing.

The value argument says: "My property is worth less than you assessed it." The equity argument says: "Even if you're right about value, you assessed me at a higher ratio than my neighbors, and that's unconstitutional." Most state constitutions and property tax codes require uniform assessment. In many jurisdictions you can win on equity alone, without proving the assessor got the value wrong.

In Illinois, for example, the Property Tax Code at 35 ILCS 200/16-135 allows a reduction if the assessment is not uniform with the median level of assessment of other properties in the same class in the same assessment district ^[2]. New York, Texas, Michigan, and most other states have their own uniformity provisions. The remedy varies. Some states reduce your assessment to the median ratio. Others grant a percentage reduction.

A private appraisal costs $300 to $600 for a typical residential property and is strong evidence of value. An equity study costs you nothing if you pull it from public data, and it's often more persuasive at a board of review because it's statistical, not subjective.

What are the key numbers in an equity study and what do they mean?

Three statistics show up in every serious equity study. Learn them before you walk into a hearing.

Median assessment ratio. The middle value when all individual ratios are ranked from lowest to highest. If the legal target is 100% and the median is 92%, the whole jurisdiction is slightly under-assessed. If you're at 108%, you're being taxed on a higher effective basis than your typical neighbor.

Coefficient of Dispersion (COD). This measures how spread out the ratios are around the median. A COD of 10 means most properties sit within about 10% of the median ratio. The IAAO recommends a COD of 15 or below for residential properties in urban and suburban areas ^[1]. A high COD is the signature of a sloppy or outdated assessment roll, where some people are badly over-assessed and others under-assessed.

Price-Related Differential (PRD). This one catches regressivity: whether expensive homes are systematically under-assessed relative to cheap ones. A PRD above 1.03 is a red flag for regressivity ^[1]. Research by the University of Chicago's Mansueto Institute found that in Cook County, the effective tax rate for the least expensive homes was more than twice that of the most expensive homes before a major reassessment reform, a finding that went straight to the PRD metric ^[3].

Here's a simple reference table:

Statistic	IAAO Acceptable Range	What it means for you
Median ratio	90%-110% of legal target	Above 110% = jurisdiction-wide over-assessment
COD (residential)	Below 15%	Above 15% = high inequality; equity appeal likely viable
PRD	0.98-1.03	Above 1.03 = cheaper homes pay higher effective rate

If your individual ratio exceeds the median ratio, that gap is your equity argument. Most hearing officers understand it once it's laid out clearly.

Who publishes assessment equity studies, and where do you find them?

There are four main sources, and they vary a lot in quality and availability.

State oversight agencies. About 35 states have an agency that monitors local assessment quality: departments of revenue, tax equalization boards, or property tax divisions. Many publish annual or biennial sales ratio studies for every county or township. These are the most authoritative data you can bring to a hearing, because the assessor can't dispute their own state's numbers. Examples include the Illinois Department of Revenue's assessment equity reports, the Minnesota Department of Revenue's Sales Ratio Studies, and the New York State Office of Real Property Tax Services annual report ^[4].

Local assessor's office. Some offices publish their own ratio studies or hand over raw sales data. Quality varies. A well-run office like the ones in Montgomery County, MD or Hennepin County, MN posts detailed performance reports online. Others post almost nothing. Check the assessor's annual report section of the county website first.

Academic and investigative studies. The ProPublica and Chicago Tribune investigation of Cook County (2017), the Lincoln Institute of Land Policy's assessment equity research, and the IAAO's own benchmarking data are all credible secondary sources. They won't have your specific neighborhood's ratio, but they can establish systemic problems that back your argument.

Your own calculation. Pull three to six recent sales of comparable properties from your county's public sales records (most counties post these, often through a GIS or assessor portal). Divide each property's assessed value by its sale price. If your ratio lands well above the group median, that's a homemade equity study. It's less powerful than a state-published study, but hearing officers in most states accept it as evidence.

For major metro areas, county-specific coverage gives you a starting point: the Cook County tax assessor tax bill article walks through how Cook County posts sales data, and the LA County property tax guide covers how California's assessment ratio rules work differently under Prop 13.

How do you formally request an assessment equity study from your assessor or state agency?

The process differs by state, but there are three main routes.

Route 1: Download a published study. For states that publish sales ratio studies, no request is needed. Minnesota's Department of Revenue posts annual Sales Ratio Studies by county at mn.gov/revenue ^[5]. Illinois posts assessment equity reports by county and township at tax.illinois.gov ^[6]. Start here. If your state has a published study, that document becomes Exhibit A in your appeal.

Route 2: Public records request. If published studies aren't available, file a public records request (called a FOIA request at the federal level, but each state has its own open records law) with the assessor's office or the state department of revenue. Ask for the sales ratio study, the sales verification data used in the most recent reassessment, or both. Most assessors have to keep this data. Frame the request specifically: ask for "the sales ratio or assessment ratio study for [county/township] covering the most recent reassessment period, including individual property ratios and summary statistics."

Route 3: Request at your state board of equalization. Many states have a formal equalization board with the authority to adjust assessments up or down across an entire class of property. Filing a complaint there, or just showing up at a public meeting and asking what ratio studies they've reviewed, is an underused tactic. The board's data is public in most states and can be cited at a local hearing.

Allow 10 to 30 business days for a records request. Some state agencies move faster. If the agency claims the data is proprietary or exempt, that's unusual for this kind of statistical summary and worth pushing back on, ideally with a letter citing your state's open records statute by number.

Can you build your own equity study with public data?

Yes, and it's simpler than it sounds. Here's how.

Start with your county's public property search tool or GIS portal. Search your neighborhood or subdivision. Most county databases show both the assessed value and, most usefully, recent sale prices. Note the sale price and assessed value for every property near you that sold in the past 12 to 24 months. Use sales no older than 24 months. Older sales carry less weight with hearing officers.

For each sale, divide the assessed value by the sale price. That's the assessment ratio for that parcel. Find the median of all those ratios. If your own property's ratio runs noticeably higher than that median, say 15% or more above it, you have a working equity argument.

Put it in a simple table: address, sale date, sale price, assessed value, ratio. Your property goes in the same table. Keep it to one page if you can. Hearing officers read dozens of cases in a day. Clean exhibits win.

One honest caveat. A handful of comps is not as statistically powerful as a 500-sale state study. You're more exposed to the assessor saying "those sales aren't truly comparable." Use the closest matches in size, age, style, and location. The stronger your comp selection, the harder that objection is to sustain.

For how individual counties handle sales data and respond to equity arguments, the Montgomery County property tax and Gwinnett County tax assessor pages give useful county-specific detail.

How do you use an equity study argument at a property tax appeal hearing?

The equity argument is a separate legal track from the value argument. Most hearing officers, especially at the informal level, are more comfortable with value arguments because that's most of what they see. Present the equity evidence clearly and in order so they can follow the logic.

Organize your evidence this way. First, cite the statute in your state that requires uniform assessment. Second, present the state or county ratio study showing the median assessment ratio for your property class and area. Third, show your individual ratio. Fourth, calculate the dollar impact: if the median ratio is 95% and yours is 112%, the assessed value should be your sale price or estimated value times 0.95, and the difference in assessed value times your tax rate equals the over-payment.

Some states hand you a bright line. In New York, the Real Property Tax Law Section 305 states that "all real property in each assessing unit shall be assessed at a uniform percentage of value" ^[7]. If a study shows you're above that percentage, the board has to correct it. In Texas, Tax Code Section 41.43 says that if the appraisal ratio of a property exceeds the median appraisal ratio of comparable properties by more than 10%, the appraisal is unequal and must be reduced ^[8]. Texas is one of the cleaner statutory setups for this argument.

Bring printed copies of everything: the state study, your comp table, the relevant statute, and a one-page summary. Leave a copy with the hearing officer. Keep your testimony under five minutes. State the median ratio, state your ratio, state the statutory requirement, and ask for the reduction to the median. Done.

If you want a structured packet for building this case yourself, the TaxFightBack DIY appeal kit walks through exactly how to assemble an equity exhibit alongside a value argument, so you're covered on both tracks without paying a contingency firm.

What ratio gap is large enough to make an equity appeal worth pursuing?

There's no universal cutoff, but a few benchmarks help.

Texas sets the bar at 10% above the median appraisal ratio of comparable properties ^[8]. That's a statutory bright line: exceed it and you win the equity argument by law. Most other states don't write a specific percentage into their statutes, leaving it to hearing officer discretion. In practice, a 5% to 10% gap in a state without a statutory threshold is arguable but not guaranteed. A 15% or larger gap is strong in almost any jurisdiction.

Look at your absolute dollar exposure too. A 10% over-assessment on a $200,000 home adds $20,000 to the assessed value. At a 2% effective tax rate, that's $400 a year. A formal appeal takes a few hours of your time. Worth it. On a $600,000 home with a 12% gap, you're looking at $72,000 in excess assessed value and possibly $1,400 or more in annual over-payment. That's easily worth a morning of prep work.

For commercial property owners, the equity argument can hit even harder, because assessment errors on income-producing properties tend to be larger in absolute dollars. The Hennepin County property tax page covers the Minnesota-specific rules on commercial equity appeals, which differ from residential.

Are assessment equity studies available in every state?

No, and the variation is wide. Some states have strong, public data. Others have almost nothing.

States with detailed published ratio studies include Minnesota, Illinois, Michigan, New York, California (though Prop 13 changes how ratios work), New Jersey, and Wisconsin. Their oversight agencies publish county-level or assessing-unit-level data annually, and you can download it directly.

States with weaker or harder-to-reach data include many Southern states, where local assessors operate with limited state oversight and don't always publish ratio statistics. That doesn't put equity arguments off the table. It means you'll likely build the study yourself from public sales data, and the statutory uniformity requirement still exists in every state's constitution or tax code.

A few states run equalization rates instead of ratio studies. California's county assessment practices study is run by the Board of Equalization and covers the aggregate, not individual properties, because Prop 13 locks most assessments to purchase price. In that world, equity arguments usually center on whether comparable properties got the same Prop 13 base year treatment, not on open-market ratios. The Santa Clara property tax article covers how that plays out in one of the state's largest counties.

The IAAO maintains a map of state oversight practices, and the Lincoln Institute of Land Policy has published comparative data on which states have the strongest assessment accountability systems ^[9].

What if the assessor refuses to provide the data or says no study exists?

Push back, in writing. Start with your state's open records law. Every state has one. File a formal written request citing the statute by name and number. In most states, assessed values, sales records used for ratio studies, and ratio study summary reports are public records not subject to exemption.

If the assessor claims no ratio study exists, that itself is worth noting at your appeal hearing. IAAO standards call for assessing offices to conduct ratio studies ^[1]. A jurisdiction that can't produce any ratio study data arguably isn't meeting professional standards, and you can say so.

Get stonewalled? Go to the state oversight agency directly. File your public records request there for statewide or county-level ratio data. The state agency isn't the same bureaucracy as the local assessor and often holds the data independently.

As a last resort, check what your local newspaper's data team has published. The ProPublica and Chicago Tribune Cook County investigation, the Detroit News coverage of Wayne County, and similar regional investigations have produced public data tables that advocates have used in hearings. The journalism exists precisely because the raw data does.

For jurisdictions with active public records environments, like New York City, the data infrastructure is deep. The NYC property tax article covers how to reach sales ratio data through the city's property information portals.

How does an equity study finding affect your final tax bill?

Win an equity argument and the hearing officer or board adjusts your assessed value down to align with the median ratio for your class of property. That reduction flows straight to your tax bill.

Here's the math. Say your home's fair market value is $350,000. The median assessment ratio in your county is 93%. Your equitably correct assessed value is $325,500. The assessor had you at $392,000, a 112% ratio. The board corrects you to $325,500. At a combined tax rate of 2.1%, you save about $1,386 a year. In most states, you can also ask for a refund of over-payments for the current tax year and sometimes the prior year. Rules on retroactivity vary by state.

The reduction usually holds until the next reassessment cycle. That's why an equity win is worth more than a single year's savings. A reduction that holds for three years in a triennial reassessment jurisdiction is worth three times the annual figure.

For states where you pay online during an appeal, keep records of payments made under protest. The online tax payment for property page covers how to flag a payment as under-protest so you keep your refund rights if you win.

Frequently asked questions

Is an assessment equity study the same thing as a sales ratio study?

Essentially yes. The terms often get used interchangeably. A sales ratio study is the data collection process: comparing assessed values to recent sale prices. An assessment equity study is the analysis and report built from that data. Some state agencies title their publication one way, some the other. Both measure whether assessed values are uniform and close to the legal target ratio.

How recent do the sales need to be to use in an equity study?

Most assessing offices and state agencies use sales from the 12 to 24 months before the assessment date. IAAO standards suggest at least one full year of sales to account for seasonal variation. Sales older than 24 months lose credibility with most hearing officers because market conditions shift. When building your own comp table, stick to sales within the 12 months before your valuation date if you can.

Can I use an equity study argument if I've never sold my home?

Yes. You don't need to have sold your property to make an equity argument. Your ratio gets calculated using either your property's recent sale price or an independent estimate of market value. You compare that ratio to the median ratio from the state or local study, which is based on other properties' sales. Your own home's sale history is irrelevant to the equity calculation.

What is the coefficient of dispersion and why does it matter for my appeal?

The coefficient of dispersion (COD) measures how much individual assessment ratios vary around the median. A COD of 20, for example, means ratios spread out widely: some properties are severely over-assessed and others under-assessed. The IAAO's acceptable COD for residential property is below 15%. A high COD in your jurisdiction backs the argument that the assessment roll is inequitable and raises the odds your particular property is a victim of that spread.

Do I need a lawyer or a professional appraiser to present an equity study argument?

Not necessarily. At informal appeal hearings and most boards of review, you can represent yourself. A published state ratio study, your individual ratio calculation, and the relevant uniformity statute are all you need to make the argument. If your case escalates to tax court, an appraiser or attorney becomes more valuable. For initial appeal levels, a clear, organized packet of evidence usually does the job without professional help.

How do I find out what the median assessment ratio is in my specific county?

Check your state department of revenue or property tax oversight agency website first. Minnesota, Illinois, New York, Michigan, New Jersey, and Wisconsin all publish county-level median ratios annually. If your state doesn't publish them, call the assessor's office and ask what ratio was used in the most recent mass appraisal. You can also calculate an informal median from public sales data in your county's property records portal.

Can commercial property owners use assessment equity studies too?

Yes, though the process runs a bit more complex. Commercial properties are typically assessed as a separate class, so the relevant median ratio is the median for commercial properties, not residential. Some states break this down further by property type: retail, office, industrial, multifamily. The same logic applies. If your commercial property's ratio exceeds the median for its class, you have a statutory equity argument. Texas Tax Code 41.43 explicitly covers commercial properties in its unequal appraisal provisions.

What happens if the state study shows the median ratio is below the legal target? Does that help me?

If the median ratio is below the legal target (say, 88% in a state requiring 100%), the whole jurisdiction is under-assessed on average. That doesn't automatically help individual owners, because equity is about your ratio relative to the median, not the median's relationship to the target. It does suggest the assessor has a systemic accuracy problem, which can support a broader argument that the entire roll needs revision.

How long does it take to get ratio study data through a public records request?

State response times vary. Most state open records laws require a response within 5 to 30 business days. Michigan's Freedom of Information Act sets a 5-business-day initial response requirement. Illinois allows up to 5 business days for non-commercial requests. If you're up against an appeal deadline, file the request immediately and also try downloading any publicly posted studies directly from the state agency's website, which is faster.

Can an equity study get my taxes raised if it shows I'm under-assessed?

Technically, an assessor who sees that someone is significantly under-assessed could correct it upward. In practice, this almost never happens because a taxpayer filed an equity appeal. The assessor's legal authority to raise your value usually runs on its own notice-and-review schedule. Filing an appeal does not waive your right to contest any upward correction, and most states limit mid-year assessment increases to specific statutory triggers.

Are assessment equity studies used for property tax appeals in all states?

The statistical equity argument is available in almost every state, because uniform assessment is a constitutional or statutory requirement everywhere. The quality and availability of published ratio data, though, varies widely. Texas has the clearest statutory threshold (10% above the median ratio triggers a mandatory reduction under Tax Code 41.43). States like Alabama or Mississippi have less developed public data, but the legal right to equitable assessment still exists under state law.

What is the price-related differential and when does it help my appeal?

The price-related differential (PRD) measures whether expensive properties get assessed at a lower ratio than cheap ones. A PRD above 1.03 indicates regressivity: lower-value homes carry a higher effective tax burden. If your home sits in the lower or middle price tier and your jurisdiction has a high PRD, that's evidence the assessment system is biased against you. Present the PRD alongside your individual ratio to strengthen the equity argument, especially for entry-level or mid-priced homes.

What is an equalization rate and how is it different from an assessment ratio?

An equalization rate is a state-calculated adjustment factor applied to a local assessment roll to bring it to a common level for distributing state aid or setting county taxes. New York State publishes equalization rates for every assessing unit annually. An assessment ratio is a property-level calculation. The equalization rate is essentially the state's official estimate of the local median assessment ratio, and it can be used directly as evidence in a New York equity appeal.

Sources

1. International Association of Assessing Officers (IAAO), Standard on Ratio Studies: IAAO standards define the primary equity measures as median assessment ratio, COD, PRD, and price-related bias index; acceptable COD for residential property is below 15%
2. Illinois General Assembly, 35 ILCS 200/16-135, Property Tax Code: Illinois law allows assessment reduction if the assessment is not uniform with the median level of assessment of other properties in the same class in the same assessment district
3. University of Chicago Mansueto Institute for Urban Innovation, Reassessing the Climb (2021): Research found that in Cook County, the effective property tax rate for the least expensive homes was more than twice that of the most expensive homes, illustrating severe PRD-related regressivity
4. New York State Office of Real Property Tax Services, Annual Assessment Equity Reports: New York State publishes annual equalization rates and assessment equity data for all assessing units in the state
5. Minnesota Department of Revenue, Sales Ratio Studies: Minnesota's Department of Revenue posts annual sales ratio studies by county, providing publicly accessible median ratios and COD figures
6. Illinois Department of Revenue, Assessment Equity Studies: Illinois posts assessment equity reports by county and township covering recent reassessment periods
7. New York State Real Property Tax Law, Section 305: NY RPTL Section 305 states that all real property in each assessing unit shall be assessed at a uniform percentage of value
8. Texas Tax Code, Section 41.43, Unequal Appraisal: Texas Tax Code Section 41.43 states that if the appraisal ratio of a property exceeds the median appraisal ratio of comparable properties by more than 10%, the appraisal is unequal and must be reduced
9. Lincoln Institute of Land Policy, Significant Features of the Property Tax: Lincoln Institute research provides comparative data on which states have the strongest assessment oversight and accountability systems
10. Michigan Freedom of Information Act, MCL 15.235: Michigan's FOIA requires a 5-business-day initial response to public records requests
11. Illinois Freedom of Information Act, 5 ILCS 140: Illinois FOIA allows up to 5 business days for response to non-commercial records requests
12. Lincoln Institute of Land Policy, Assessment Equity in the United States: A 50-State Comparison: Comparative study documenting variation in assessment equity practices and data availability across all 50 states