What Is Sales Ratio Study
A sales ratio study analyzes the relationship between assessed values and actual sale prices in a jurisdiction to measure assessment uniformity. Tax assessors and appraisers use these studies to determine whether properties across a district are assessed at consistent levels relative to their market value.
The core calculation is straightforward: divide the assessed value by the sale price, then average the ratios across all comparable sales. If homes in your county sell for an average of $300,000 but assessments average $210,000, the assessment ratio is 70%. That same study might reveal some neighborhoods cluster at 65% while others hit 75%, signaling unequal treatment.
This matters directly to your property tax bill. Assessment ratios vary significantly by county and state. Some jurisdictions assess at 100% of market value, others at 50% or lower. What matters for your appeal is whether your property is assessed at a materially different ratio than comparable properties in your area.
How Assessors Use Them
Assessors conduct sales ratio studies annually or biannually to audit their own work. When a study shows an assessment ratio of 65% districtwide but your property is assessed at 80%, that discrepancy becomes evidence of over-assessment. Conversely, if the study shows 75% across the board and your assessment is at 75%, your position weakens in a board of review hearing.
Counties report these studies to state equalization boards. State regulations in some jurisdictions (like New York and Illinois) require assessors to maintain assessment ratios within specific ranges. Illinois targets 28-35% of market value for residential property. If your county's study shows systematic deviation from that band, it triggers equalization adjustments that affect all properties.
Using Sales Ratio Studies in Appeals
- Benchmark your property: Request your county's most recent sales ratio study. Compare your assessment ratio to the districtwide average. A property assessed at 85% when the study shows 70% gives you concrete evidence for appeal.
- Identify comparable sales: Sales ratio studies typically exclude foreclosures, short sales, and non-arm's-length transactions. Use the same property types and recent timeframe (last 12-24 months) when selecting comps for your appraisal method.
- Present findings at board of review hearings: Bring the official study to your hearing. Show the assessment ratio discrepancy alongside your own comparable sales analysis. The study becomes third-party validation of market-based values.
- Challenge district-wide assessments: If the study reveals systematic over-assessment across your neighborhood or property class, push for a broader equalization correction rather than a single-property adjustment.
Key Data Points
Most counties publish assessment ratio statistics including the median ratio, coefficient of dispersion (which measures uniformity), and breakdowns by property type and neighborhood. A coefficient of dispersion below 15% indicates good uniformity; above 20% signals problematic inconsistency and strengthens your case for reassessment.
Request studies covering at least 30-40 recent sales in your class and area. Smaller sample sizes produce unreliable ratios. Also ask for the study's methodology: did they exclude new construction, exempt properties, or specific price ranges? These choices affect the results.
Common Questions
- Can I request a copy of my county's sales ratio study? Yes. Most states require public access. Contact your assessor's office or county clerk. Some publish studies online. Request the most recent complete study, not just summary statistics.
- What if the sales ratio study shows my assessment is actually below the average? Your appeal position weakens, but the study may reveal other evidence: perhaps your neighborhood is assessed lower than adjacent ones, or a specific property class is over-assessed. A skilled appraiser can still find grounds for adjustment using the study's own data against itself.
- How recent does the sales ratio study need to be? Use the latest available study, typically from the previous year. If your assessment notice is from 2024, a 2023 study is current. Studies more than two years old may not reflect current market conditions, though older studies can still support systematic under-assessment arguments.
Related Concepts
- Coefficient Of Dispersion measures the statistical spread of assessment ratios and indicates whether your county assesses uniformly.
- Equalization Rate is the average assessment ratio used by states to adjust property tax levies across districts with different assessment practices.