When your county assessor determines the value of your property, they're supposed to do it fairly. Not just fairly for you, but fairly for everyone in the county. The Coefficient of Dispersion — or COD — is the single most important number that tells you whether they're getting it right.
Assessment Equity in Plain Language
The principle behind assessment equity is simple: if two houses are worth the same amount, they should be assessed the same amount. If one house is worth twice as much as another, it should be assessed at roughly twice as much.
In practice, this is hard to achieve. County assessors must value thousands or even hundreds of thousands of properties using limited data, and the assessment process involves judgment calls at every step. Perfect accuracy is impossible. The question is: how far off are they, and are the errors random or systematic?
That's what the COD measures.
How the COD Is Calculated
The COD is calculated using assessment-to-sale ratios. When a property sells, we can compare its assessed value to its sale price to get a ratio:
Assessment Ratio = Assessed Value / Sale Price
If a property assessed at $150,000 sells for $160,000, the ratio is 0.9375 (slightly under-assessed). If it sells for $140,000, the ratio is 1.0714 (slightly over-assessed).
The COD measures how much these ratios vary across all sales in a jurisdiction. Technically, it's the average absolute deviation of the ratios from the median ratio, divided by the median ratio, expressed as a percentage.
A COD of 0 would mean every property is assessed at exactly the same percentage of market value — perfect uniformity. Higher COD values indicate more dispersion — some properties are over-assessed relative to others.
What's a Good COD?
The International Association of Assessing Officers (IAAO) establishes standards for acceptable COD levels:
| Property Type | IAAO Standard |
|---|---|
| Single-family residential (newer, homogeneous areas) | 5.0 - 10.0 |
| Single-family residential (older, heterogeneous areas) | 5.0 - 15.0 |
| Income-producing property | 5.0 - 20.0 |
| Vacant land | 5.0 - 25.0 |
For most Indiana residential property, a COD below 15 is considered acceptable. Below 10 is good. Below 5 is excellent.
A COD above 15 means the county has significant assessment uniformity problems. Some properties are being assessed much higher (relative to market value) than others — and the property owners paying more than their fair share are effectively subsidizing those paying less.
Why COD Matters to Homeowners
If your county has a high COD, it means assessment accuracy is a crapshoot. Your neighbor's home might be assessed at 85% of market value while yours is assessed at 110%. You'd both owe very different tax amounts on homes that should bear similar tax burdens.
This is why checking your individual assessment against comparable sales matters — especially in counties with known equity issues. Our Tax Appeal Automation product compares your assessment against comparable sales data to identify whether you're on the wrong side of the distribution.
Even in counties with good overall COD scores, individual properties can be over-assessed. The COD tells you the overall quality of the assessment roll, but your individual situation may differ.
Beyond COD: Other Equity Metrics
Price-Related Differential (PRD)
The PRD measures whether assessment accuracy varies by property value. A PRD of 1.00 means high-value and low-value properties are assessed equally accurately. A PRD above 1.03 indicates regressivity — lower-value properties are assessed at a higher percentage of market value than higher-value properties.
Regressive assessment is a serious equity concern because it means lower-income homeowners bear a disproportionate share of the property tax burden. This pattern is common nationally and has been documented across many Indiana counties.
Price-Related Bias (PRB)
The PRB is a more sophisticated test of the relationship between assessment accuracy and property value. It detects bias that the PRD might miss by using a regression approach rather than a simple ratio.
Median Assessment Ratio
The median ratio tells you whether the county is generally over- or under-assessing relative to market value. While the COD measures dispersion (uniformity), the median ratio measures the level of assessment.
Indiana targets assessments at 100% of market value in use. A median ratio significantly above 1.00 means the county is systematically over-assessing, while below 1.00 means under-assessing.
Sales Chasing: A Hidden Problem
Sales chasing occurs when an assessor adjusts the assessed value of a property specifically because it sold, while leaving comparable unsold properties unchanged. This distorts the assessment-to-sale ratio statistics and can make a county's COD look better than it actually is.
Detecting sales chasing requires comparing assessment accuracy between recently-sold properties and unsold properties. If sold properties have significantly better ratios than unsold properties, sales chasing is likely occurring.
Our Assessment Equity Audits product includes statistical tests specifically designed to detect sales chasing patterns.
Why County Assessors Should Monitor COD
Indiana county assessors face scrutiny from multiple directions:
DLGF Compliance
The Indiana Department of Local Government Finance (DLGF) monitors assessment equity across the state. Counties with poor equity metrics can face oversight actions, including required reassessments at county expense.
Taxpayer Appeals
Counties with high COD scores tend to face more Form 130 appeals because more individual assessments are inaccurate. Each appeal consumes staff time and PTABOA resources.
Public Trust
When assessment inequity is documented — whether by journalists, advocacy groups, or platforms like AribaTax — public trust in the assessment process erodes. Proactive monitoring and correction of equity issues is better than reactive defense.
Constitutional Requirement
Indiana's constitution requires that property assessment be "uniform and equal." A high COD is evidence that this standard isn't being met.
How AribaTax Helps
For Homeowners
Use our Property Lookup to check your assessment against comparable sales. If you appear over-assessed, our Tax Appeal Automation can handle the appeal process.
Our AI Valuation product provides an independent estimate of your property's market value using models trained exclusively on Indiana sales data. Comparing this estimate against your assessment gives you a data-driven basis for deciding whether to appeal.
For County Assessors
Our Assessment Equity Audits product delivers IAAO-compliant ratio studies for all 92 Indiana counties. The reports include:
- COD, PRD, and PRB analysis at county, township, neighborhood, and property class levels
- Sales chasing detection
- Year-over-year trending
- Peer county benchmarking
- Demographic disparity analysis
- Print-ready PDF reports for DLGF compliance and county council presentations
Reports update automatically as new sales and assessment data arrives. Schedule a demo or contact us to learn more.
Explore Assessment Data
Browse assessment data for specific Indiana counties:
- Marion County — 350,000+ parcels, Indiana's largest assessment roll
- Lake County — Complex multi-city assessment landscape
- Allen County — Strong CAMA data, good assessment practices
- Hamilton County — High-value residential market
- St. Joseph County — South Bend metro area