Last updated 2026-07-10
TL;DR
A sales ratio study compares recent sale prices to assessed values across a jurisdiction. Assessors use them to prove mass appraisal accuracy; taxpayers can use the same data to prove their property is overassessed relative to neighbors. Most states require assessments to fall within 90-110% of market value. A ratio outside that band is grounds for appeal, sometimes without any comparable sales of your own.
What is a sales ratio study and why do assessors run them?
A sales ratio study is a statistical audit. The assessor takes a batch of arm's-length property sales from a defined period, divides each property's assessed value by its sale price, and computes statistics across the whole batch. That ratio, assessed value divided by sale price, is called the assessment-to-sales ratio, or simply the sales ratio. [1]
The math is plain. If a house sold for $400,000 and the assessment is $360,000, the sales ratio is 0.90, or 90%. Do that calculation for every qualified sale in the county, sort the results, and you get a distribution that tells you whether the assessor's mass appraisal model is hitting its target.
Assessors don't run these studies out of curiosity. State law usually requires them. The International Association of Assessing Officers (IAAO) publishes the standard most states borrow from: its Standard on Ratio Studies sets acceptable bounds for the median assessment-to-sales ratio, the coefficient of dispersion (COD), and the price-related differential (PRD). [2] Many state departments of revenue or equalization boards commission independent ratio studies to check local assessors' work, and those results can trigger forced reassessments or equalization adjustments when a county drifts far off target.
So when you hear that your county is "equalized at 100%" or "assessed at 33.33% of market value," that figure came from a ratio study. It's the official answer to one question: how close are we, on average, to what properties would actually sell for?
How is the sales ratio actually calculated?
The basic formula is:
Sales Ratio = Assessed Value / Sale Price
That ratio gets computed for every arm's-length sale in the study period, typically the prior calendar year or the 12-18 months before the assessment date. Sales that aren't arm's-length (foreclosures, sales between relatives, deeds in lieu of foreclosure) get thrown out. What's left is the study universe.
From that universe, assessors calculate several statistics:
| Statistic | What it measures | IAAO standard for residential |
|---|---|---|
| Median ratio | The midpoint assessment-to-sales ratio | 0.90 to 1.10 (90% to 110%) [2] |
| Coefficient of Dispersion (COD) | How widely individual ratios scatter around the median | Below 15% (single-family residential) [2] |
| Price-Related Differential (PRD) | Whether expensive homes are assessed lower relative to cheap ones | 0.98 to 1.03 [2] |
| Mean ratio | The average of all ratios; compared to median to spot outliers | No hard standard; used diagnostically |
The median ratio is the headline number. A median of 0.95 means the typical assessment sits 5% below market. A median of 1.08 means assessments run 8% above market. Both fall inside the IAAO band. Only one hurts you.
The COD matters because it measures fairness between neighbors. A low median can hide a high COD where cheap houses get assessed at 120% of value and expensive ones at 80%. The PRD is built to catch exactly that regressivity pattern. [2]
State agencies publish these numbers. Illinois publishes township-level median ratios and CODs through the Illinois Department of Revenue. [3] Minnesota's Department of Revenue publishes annual sales ratio reports by county and property class. [4] Find your jurisdiction's numbers and you already know whether the system is running hot or cold, before you ever look at your own assessment.
What does the sales ratio mean for your specific assessment?
Here's where it gets personal. The county-wide median ratio tells you the average story. Your property's individual ratio tells you yours.
To find your ratio: take your assessed value, divide by what you believe the property is worth right now. If your assessed value is $520,000 and you think the house is worth $460,000, your individual ratio is 1.13. You're assessed 13% above market.
Now compare that to the county median. If the county median ratio is 0.96, you're sitting 17 percentage points above the county norm. That gap is the core of an overassessment argument, even if you can't point to a single comparable sale.
Many states allow appeals on uniformity or equalization grounds, meaning your assessment is unfair relative to other properties even if it's close to market value. The legal standard varies by state. New York lets taxpayers contest on grounds of "inequality," which Real Property Tax Law Section 701 defines as an assessment that is a higher percentage of value than the assessments of comparable residential properties in the same assessing unit. [5] You don't have to prove the assessor got the market value wrong. You prove your ratio is out of line with the median.
This is the part most homeowners miss. They fight the absolute number when the ratio comparison is a cleaner, faster argument.
How do assessors use ratio studies to defend their assessments at a hearing?
When you show up at an appeal board and say "my house is overvalued," the assessor's standard defense is the ratio study. They'll produce the median ratio and argue their mass appraisal model sits within IAAO standards, so any individual divergence is acceptable statistical noise.
This is a real argument, and it works against unprepared appellants. If the county median ratio is 0.98 and you're asking the board to believe your specific house is assessed 25% above market, the assessor will push hard on your evidence quality. You need comps that actually support your claim, or you need to show that your property's class, neighborhood, or price tier carries a meaningfully different ratio than the county median.
Assessors also use ratio studies internally to calibrate their mass appraisal models between full reappraisal cycles. When sales data shows the median ratio drifting toward 1.10 or above (assessments catching up to a falling market), many counties apply a blanket equalization factor rather than reassess every parcel. Cook County, Illinois works within an equalization system where the Illinois Department of Revenue sets an equalization factor (the "multiplier") to bring assessed values to 33.33% of market value statewide. [3] That multiplier comes straight out of ratio studies.
So ratio studies cut both ways. When the study shows the assessor's numbers are good, it shields the assessor at your hearing. When the study shows a high median ratio or a high COD in your neighborhood, it becomes your best evidence.
How can you find and use sales ratio data in your own appeal?
Start with your state. Most state departments of revenue, equalization, or taxation publish annual ratio studies. A few real examples:
- Minnesota Department of Revenue publishes annual sales ratio reports by county and property class going back years. [4]
- Illinois Department of Revenue publishes median ratios by township each year, which is exactly what you'd bring to a Cook County appeal. [3]
- New Jersey Division of Taxation publishes an annual Table of Equalized Valuations by county. [6]
- The IAAO maintains a resource library, and many states mirror its methodology documentation. [2]
Once you have the aggregate data, dig into the parcel-level sales your county likely publishes. Most assessor websites let you download recent sales by neighborhood or property class. Build your own mini-ratio study for your immediate area. Pull 20 sales within a quarter-mile of your house, find the median ratio is 1.11, and that neighborhood-level evidence beats the county median at your hearing.
You're hunting for three things in the data:
1. A median ratio above the state's standard (often 1.00 or a set assessment level like 33.33% in Illinois) means assessments are systematically high. 2. A high COD in your neighborhood means the assessment is uneven, which supports a uniformity argument. 3. Properties like yours (same size, age, condition, street) carrying lower ratios than yours directly support your individual appeal.
If you want a structured process for pulling this evidence together, the TaxFightBack DIY appeal kit walks through exactly this methodology, including how to build a neighborhood ratio table and present it at a hearing.
For homeowners in specific high-volume jurisdictions, the local rules matter enormously. Cook County and LA County both publish parcel-level sales data that feeds directly into this kind of analysis. Montgomery County in Maryland posts assessment ratio documentation alongside individual notices, which makes the comparison unusually straightforward.
What is a coefficient of dispersion and why should you care about it?
The coefficient of dispersion (COD) measures horizontal equity. Horizontal equity means similar properties should get assessed at similar ratios. A COD of 10% means individual property ratios scatter within roughly plus or minus 10 percentage points of the median. A COD of 25% means the system is far less predictable, with some owners paying on assessments 25% or more above or below their neighbors' ratios. [2]
The IAAO standard for single-family residential properties in larger, more urban jurisdictions is a COD below 15%. For rural areas with thinner sales data, the standard eases to a limit of 20%. [2]
Why does this matter to you personally? Because a high COD is an equity argument that's hard for an assessor to wave away. Show the board that your neighborhood's COD is 22% and your ratio sits in the top quartile of that dispersion, and you're not asking for special treatment. You're asking to be moved to the median. That's a defensible position even if the assessor argues your absolute market value is roughly right.
The PRD (price-related differential) is a companion statistic. A PRD above 1.03 means lower-value properties get assessed at systematically higher ratios than higher-value ones. That's regressive assessment, and it's a documented problem in many urban jurisdictions. A 2017 investigation by ProPublica and WBEZ Chicago found significant regressivity in Cook County's assessments before a later reform process began. [7] Regressivity doesn't automatically win your individual appeal, but it sets a statistical context that hearing boards are increasingly aware of.
What is an equalization factor and how does it connect to ratio studies?
An equalization factor (sometimes called a multiplier) is a correction applied to all assessed values in a jurisdiction when the ratio study shows the median is well off target. If a state mandates assessments at 100% of market value and the ratio study shows a county assessing at a median of 85%, the state can apply an equalization factor of 1.176 (1 divided by 0.85) to bring every assessed value up to the statewide standard.
This matters for your appeal in two ways.
First, if an equalization factor has already been applied to your county, your assessment may look higher than the assessor's own original figures suggested. The factor is mechanical. It doesn't account for your specific property's condition or market position. You can still appeal the equalized value.
Second, if you're in a state where equalization factors get recalculated each year, the timing of your appeal matters. File before the factor is set and you're appealing the raw assessment. File after and you're appealing the equalized value. Check your state's assessment calendar.
Illinois is the textbook example. The state sets an assessment level of 33.33% of market value for all property. The Department of Revenue calculates each county's multiplier from ratio studies and publishes it. [3] In Cook County, the multiplier has historically landed between roughly 2.6 and 3.0 depending on where the median ratio falls. The local assessor assesses at a target of 10% of market value, and the state multiplier brings it up to the statutory 33.33%.
Can you use a bad sales ratio study to challenge your entire jurisdiction's assessment?
In theory, yes. In practice, it's a different kind of fight.
If a ratio study shows systematic overassessment (a median ratio well above the statutory target), property owners can, and sometimes do, file class-action-style challenges or join organized petition campaigns. In states with strong uniformity provisions, showing that the entire class of residential properties in a township is assessed above the statutory level can compel a rollback for all affected parcels.
But individual homeowners rarely go this route alone. It takes legal standing, expert testimony, and often a mass of individual appeal filings that politically forces the assessor's office to negotiate. The more realistic individual use of a bad ratio study is as supporting evidence in your personal appeal, to show that the assessor's own data reveals a systemic problem.
At your hearing, say something like: "The state's ratio study for this county shows a median assessment-to-sales ratio of 1.09. My property's ratio is 1.17. I'm asking the board to bring my assessment to the county median." That's a concrete, data-grounded ask, and it's much harder to refuse than a vague "I think it's too high."
For homeowners in jurisdictions with published parcel-level data, such as Gwinnett County or Hennepin County, the county's own sales data is free, and you can build a credible mini-study yourself, without hiring an expert.
What states publish ratio studies that homeowners can actually access?
Most states publish ratio studies in some form. The depth and timeliness vary a lot.
| State | Who publishes | How detailed | Where to find |
|---|---|---|---|
| Illinois | IL Dept. of Revenue | Township-level median ratios and multipliers | tax.illinois.gov [3] |
| Minnesota | MN Dept. of Revenue | County and class-level annual reports | revenue.state.mn.us [4] |
| New Jersey | NJ Division of Taxation | County equalization table | nj.gov/treasury/taxation [6] |
| New York | NY Office of Real Property Tax Services | Municipality-level equalization rates | tax.ny.gov [5] |
| California | CA State Board of Equalization | County ratio studies used for intercounty equalization | boe.ca.gov [8] |
| Texas | TX Comptroller | Annual property value study by appraisal district | comptroller.texas.gov [9] |
Texas is worth a special mention. The Texas Comptroller publishes an annual property value study for every appraisal district, and if a district's median ratio falls outside 95-105% of market value, the state can penalize the district financially. [9] That creates real pressure on Texas appraisal districts to stay accurate. If your district fails the study, every taxpayer in it has statistical ammunition for an appeal.
California is more complicated because of Proposition 13, which caps assessed value increases at 2% per year regardless of the ratio study. The ratio study there feeds intercounty school funding equalization, not individual appeals. [8] For Santa Clara or LA County homeowners, the path to appeal runs through base-year value challenges, not ratio arguments.
For New York, the Office of Real Property Tax Services publishes equalization rates by municipality. These are the official state findings on the ratio between assessments and market values for each town. [5] If your town's equalization rate is 85% and your assessment implies a ratio of 100%, you have a textbook inequality claim under RPTL Section 701.
What is the IAAO standard and does it actually protect you?
The IAAO standard says residential assessments should have a median ratio between 0.90 and 1.10, a COD below 15% (or 20% in rural areas), and a PRD between 0.98 and 1.03. [2] Many states write these numbers into statute or administrative rule, so they carry legal weight, more than professional weight.
But the IAAO standard protects you only if your state enforces it. In states with weak oversight, an assessor can produce a ratio study showing a COD of 25% and face nothing worse than a stern letter from the state department of revenue. You'd still have to appeal parcel by parcel.
The IAAO itself has no enforcement authority. It's a professional association that sets best practices. The Standard on Ratio Studies says, in its own words, that "the primary purpose of ratio studies is to provide information for the improvement of the assessment process." [2] That's practitioner language, not taxpayer-rights language. The real protection comes from state law that mandates the standard and gives taxpayers a right to cite deviations at appeal.
If you're in Texas and the comptroller's study shows your district at a median ratio of 1.10 or above, you can cite that directly at your Appraisal Review Board hearing. [9] If you're in Illinois and the state's township median ratio for your area is 1.05, that's evidence the assessor themselves produced showing systematic overassessment. These are real pressure points, not abstract statistics.
The TaxFightBack approach, across our guides including this one, is to treat the ratio study as the first document you pull, before you look at comps, before you hire anyone. It tells you whether you have a case and what kind of case you have.
How do you present ratio study evidence at an appeal hearing?
Keep it short and visual. Hearing boards see dozens of cases a day. A one-page summary table lands better than a 10-page statistical exposition.
Your summary should show:
1. The source of the ratio data (state agency name, year of study, URL or publication date). 2. The county or township median ratio. 3. Your property's calculated ratio (your assessed value divided by your estimated market value, backed by at least one comp or an appraisal). 4. The gap between your ratio and the median. 5. The specific relief you're requesting: bring my assessed value to the county median ratio applied to my market value estimate.
If the county median ratio is 0.97 and you believe your market value is $380,000, the "correct" assessment at that median works out to $368,600. If you're currently assessed at $430,000 (a ratio of 1.13), you're asking for a reduction to $368,600 or thereabouts.
Present the math clearly. Board members aren't always statisticians. Aim for a simple narrative: the county's own data says 97 cents of assessment per dollar of value is the norm; my property is assessed at $1.13 per dollar. I'm asking to be treated like everyone else.
Anticipate the assessor's counter. They may argue your comparable sale isn't truly comparable, or that your market value estimate is too low. Bring backup comps. Add a neighborhood-level ratio table (five to ten nearby sales with individual ratios all below yours) and the argument for your specific reduction gets much harder to dismiss.
For jurisdictions like Bexar County or Bibb County, the assessor's own sales data is the raw material for this neighborhood table. You're using their data against an application of their model that happened to disadvantage your parcel.
Frequently asked questions
What is a sales ratio study in property tax?
A sales ratio study compares each property's assessed value to its actual sale price (assessed value divided by sale price) across a batch of recent arm's-length sales. Assessors run these studies to measure mass appraisal accuracy. States use the same studies to check local assessors and set equalization factors. The IAAO standard says the median ratio should fall between 90% and 110% of market value.
What is a good or acceptable assessment-to-sales ratio?
The IAAO Standard on Ratio Studies sets the acceptable median assessment-to-sales ratio at 0.90 to 1.10, meaning assessments should be between 90% and 110% of market value. Many states write this range into law. A ratio below 0.90 means properties are underassessed; above 1.10 means overassessed. Your state may set a different statutory assessment level (Illinois targets 33.33%), but the 90-110% band applies around whatever that target is.
Can I use a sales ratio study to appeal my property tax assessment?
Yes. If the state or county ratio study shows a median ratio well above 1.00 (or above the statutory assessment level), you can cite it as evidence of systematic overassessment. More powerfully, if you calculate your own property's ratio and it lands well above the county median, you have a uniformity or inequality argument: you're assessed at a higher percentage of value than your neighbors, which most states treat as grounds for reduction.
What is a coefficient of dispersion in a ratio study?
The coefficient of dispersion (COD) measures how widely individual assessment ratios scatter around the median. A COD of 10% means most properties are assessed within roughly 10 percentage points of the median ratio. A COD above 15% for residential properties exceeds the IAAO standard and signals unfair, uneven assessment across the jurisdiction. A high COD in your neighborhood supports an equity-based appeal even if your absolute market value is close to the assessment.
What is an equalization factor and how is it calculated from ratio studies?
An equalization factor is a statewide or regional multiplier applied to all assessments in a jurisdiction to bring them to the statutory level. It's calculated by dividing the statutory assessment percentage by the jurisdiction's median ratio. Illinois is the most prominent example: if a township's ratio study shows a median ratio of 10.5% against a 10% target, the multiplier adjusts every parcel upward by a factor of roughly 0.952. The factor appears on your assessment notice.
Where can I find the sales ratio study for my county?
Check your state's department of revenue, department of taxation, or state board of equalization website. Texas publishes district-level studies through the Comptroller's office. Minnesota and Illinois publish township- and county-level reports annually. New York publishes municipality-level equalization rates through the Office of Real Property Tax Services. New Jersey publishes a county equalization table. Search for "[your state] sales ratio study" or "[your state] property value study."
What is the price-related differential (PRD) and what does it mean for homeowners?
The PRD measures whether lower-value properties are assessed at systematically higher ratios than higher-value ones. A PRD above 1.03 indicates regressivity: cheaper homes pay a larger share of their true value in taxes than expensive ones. The IAAO standard is a PRD between 0.98 and 1.03. If your jurisdiction has a PRD above 1.03, lower-value homeowners may have a strong equity argument, though winning on PRD alone in an individual appeal is difficult without supporting comparable evidence.
Does a low county median ratio mean my assessment is fair?
Not necessarily. A county median ratio of 0.95 means the typical property is assessed at 95% of market value, which looks fine. But a high COD means the typical story hides wide individual variation. Your property could carry a ratio of 1.15 even while the county median is 0.95. Always calculate your own ratio and compare it to the median. The median protects the average taxpayer; it doesn't protect you specifically unless your ratio is also near the median.
What evidence do I need to show my ratio is out of line at a hearing?
You need three things: the published county or state median ratio (from the official ratio study), your own market value estimate backed by at least one recent comparable sale or an appraisal, and your calculated ratio (assessed value divided by your estimated market value). If you can add a neighborhood mini-ratio table showing five to ten nearby sales with their individual ratios, all below yours, the argument is significantly stronger and harder for the assessor to dismiss.
How does Texas use ratio studies differently from other states?
The Texas Comptroller publishes an annual property value study for every appraisal district. If a district's findings fall outside 95-105% of the state's independently estimated market value, the state can reduce the district's school funding allocation. This financial penalty creates strong pressure on Texas appraisal districts to stay accurate. If your district fails the study, you can cite the Comptroller's own findings at an Appraisal Review Board hearing as evidence of systematic overvaluation.
Does California use sales ratio studies for individual property tax appeals?
Not for most homeowners. California's Proposition 13 caps assessed value increases at 2% per year from the base-year acquisition value, which makes the sales ratio mostly irrelevant for individual appeals. The State Board of Equalization does conduct ratio studies, but they're used for intercounty equalization of school funding rather than for individual taxpayer appeals. California appeals focus almost entirely on whether the assessor correctly set the base-year value or on decline-in-value claims under Proposition 8.
What is the difference between an equalization rate and an assessment ratio?
They measure the same thing from different directions. An assessment ratio is assessed value divided by market value (a number like 0.95 or 95%). An equalization rate is the state's official finding of that ratio for a municipality, expressed in percent. New York publishes equalization rates by town through ORPTS. If your town's equalization rate is 80%, the state is saying assessments there are set at 80% of market value on average. You use it to convert assessed values to estimated market values for comparison.
Can the whole neighborhood appeal together using a bad ratio study?
Informally, yes. If a ratio study shows a neighborhood or property class is systematically overassessed, organized groups of homeowners filing appeals at once can force the assessor's office to negotiate or settle cases in bulk. This happens regularly in Cook County, Illinois and in some New York jurisdictions. A formal class action on property tax is rare and legally complex, but a coordinated wave of individual appeals citing the same ratio study evidence is a well-established tactic with real track records of success.
How often are sales ratio studies updated?
Most states update their ratio studies annually, using the prior calendar year's arm's-length sales. Some states use a rolling 18- or 24-month window to gather enough sales volume in thinner markets. The study usually publishes several months after the data period closes, so the most recent published study may be six to eighteen months behind current market conditions. In a fast-moving market, that lag matters: a rising market means recent assessments may understate current values, and a falling market means they may overstate them.
Sources
- International Association of Assessing Officers, Glossary for Property Appraisal and Assessment: Definition of assessment-to-sales ratio as assessed value divided by sale price
- International Association of Assessing Officers, Standard on Ratio Studies (2013): IAAO standards: median ratio 0.90-1.10, COD below 15% residential, PRD 0.98-1.03; primary purpose is improvement of the assessment process
- Illinois Department of Revenue, Property Tax Assessment and Equalization: Illinois publishes township-level median ratios and equalization multipliers; statutory assessment level is 33.33% of market value
- Minnesota Department of Revenue, Sales Ratio Study Reports: Minnesota publishes annual county- and class-level sales ratio reports
- New York Office of Real Property Tax Services, Equalization Rates: NY ORPTS publishes municipality-level equalization rates; RPTL Section 701 defines inequality as assessment at a higher percentage of value than comparable residential properties
- New Jersey Division of Taxation, Table of Equalized Valuations: New Jersey publishes annual county equalization table derived from ratio studies
- ProPublica / WBEZ Chicago, The Tax Divide (2017): 2017 investigation found significant regressivity in Cook County assessments, with lower-value properties assessed at higher ratios than higher-value ones
- California State Board of Equalization, Property Tax Rules and Publications: California BOE conducts ratio studies for intercounty equalization; Proposition 13 limits annual assessment increases to 2% from base-year value
- Texas Comptroller of Public Accounts, Property Value Study: Texas Comptroller publishes annual property value study by appraisal district; districts outside 95-105% of estimated market value face school funding penalties