IAAO Standards: Ratio Studies & Property Tax Policy

Summary

Two short, freely downloadable standards published by the International Association of Assessing Officers (IAAO) are the profession's own definitional source for the assessment-quality vocabulary this wiki's assessability coverage relies on — currently cited mostly at second hand, through Lars Doucet's essay or through Hefferan & Boyd's appeal-rate benchmark:

• Standard on Ratio Studies (approved April 2013, ISBN 978-0-88329-208-2) — defines how a ratio study tests appraisal accuracy and uniformity, and sets out the coefficient of dispersion (COD), price-related differential (PRD), and price-related bias (PRB) metrics and their acceptable-quality ranges.[1]
• Standard on Property Tax Policy (revised and approved July 2020, "a complete revision" of the 2010 edition[2]) — states the profession's "core principles" for how a property tax system should be designed and run.[2]

Both were fetched and read in full (not summarized secondhand) for this page (2026-07-18).

What the Ratio Study Standard Defines

A ratio study compares appraised values to independently known market values (usually sale prices) to test two things: level (are values, overall, close to 100% of market value?) and uniformity (are similar properties appraised at similarly close-to-market ratios, without systematic bias toward high- or low-value parcels?).[1] It is IAAO's quality-assurance instrument for mass appraisal, used both by local assessors ("internal" ratio studies) and by state/provincial oversight agencies performing equalization.[1]

COD, PRD, and PRB — the three metrics the wiki's assessment-quality claims rest on

• Coefficient of Dispersion (COD) measures horizontal uniformity: the average percentage deviation of individual assessment ratios from the median ratio. It is computed by taking the absolute deviation of each ratio from the median, averaging those deviations, dividing by the median, and multiplying by 100.[1] "More than half the ratios fall within one COD of the median."[1]
• Price-Related Differential (PRD) measures vertical equity — whether low-value and high-value properties are assessed at the same fraction of market value. It is the mean ratio divided by the weighted mean ratio; PRDs "should be close to 1.00," with values above 1.00 indicating assessment regressivity (low-value properties over-assessed relative to high-value ones) and values below 1.00 indicating progressivity.[1]
• Price-Related Bias (PRB) is a newer, regression-based alternative to the PRD, "less sensitive to atypical prices and ratios": it regresses the percentage deviation of the ratio from the median against the percentage difference in value.[1] "As a general matter, the PRB coefficient should fall between –0.05 and 0.05"; a PRB whose 95% confidence interval falls outside ±0.10 indicates "unacceptable vertical inequities."[1]

These three metrics are the technical apparatus behind claims already on the wiki that use looser language — most directly, Berry (2021)'s finding (cited on the mass-appraisal-methods concept page) that US assessments are "typically regressive" is, in IAAO's own vocabulary, a PRD/PRB finding.

Acceptable-quality ranges — and the honest nuance for land specifically

The standard's Table 1-3 sets COD ranges by property type, and this is where it bears directly on the assessability objection: it publishes a separate, wider tolerance band for vacant land than for improved residential property.

A companion table for oversight (equalization) ratio studies stratifies further by jurisdiction size and market activity and shows the same pattern: "Residential vacant land" tolerates CODs up to 20.0–25.0 depending on jurisdiction size, and "Other (non-agricultural) vacant land" up to 25.0–30.0 — wider than the corresponding improved-property bands in every size class.[1] PRDs, by contrast, carry one uniform standard across property types: "between 0.98 and 1.03."[1] PRBs should "generally fall between −.05 and .05," with values outside ±0.10 unacceptable, again without a separate, looser band for land.[1]

This qualifies, without refuting, the wiki's "land is easier to assess" claim. Doucet and the mass-appraisal-methods page argue land value is spatially smooth and therefore no harder — potentially easier — to estimate than the heterogeneous building component. The IAAO's own quality bands do not straightforwardly confirm that: the profession expects, and tolerates, more dispersion in vacant-land ratio studies than in improved-residential ones. The more plausible reading is not that land is intrinsically harder to value, but that vacant-land sales are thinner — fewer transactions, smaller samples, more idiosyncratic parcels (irregular shapes, differing zoning, undeveloped-lot outliers) — which mechanically widens a ratio study's dispersion even when the underlying valuation method is sound; the standard's own oversight table shows the same COD band widening for any property type as jurisdiction size and market activity fall, not for land alone.[1] Still, this is a first-hand data point the wiki should carry honestly rather than only citing evidence that supports the "land is easier" framing.

The standard also explicitly contemplates land-only ratio studies: "for a ratio study of vacant land, the numerator in the ratio should reflect only the appraised value of the land. The sale price should be adjusted for the contributory value of the improvements or the sample should be excluded from further analysis."[1] This is a direct, professional-consensus confirmation that isolating and testing land value alone is routine, standardized practice — not an ad hoc Georgist workaround.

Not found: an appeal-rate / "2%" benchmark

The wiki's land-cannot-be-assessed objection page and the mass-appraisal-methods concept page cite Hefferan & Boyd for the claim that Australian appeal rates run "well below the internationally accepted 2% benchmark." The Standard on Ratio Studies was read in full for this page and contains no explicit 2% appeal-rate benchmark; that figure's source remains Hefferan & Boyd, not this IAAO standard. Readers should not infer the 2% figure is an IAAO-standard number — it is not corrected or added to here, only checked and found absent.

What the Property Tax Policy Standard Says — and Doesn't Say — About Site-Value Taxation

The Standard on Property Tax Policy states four "core principles of property tax policy":

"Assessments based on market value with regular and frequent (preferably annual) updates[;] A broad tax base with limited exemptions[;] Relief programs that are targeted, easily accessed by those who need assistance, and administratively efficient[;] An assessment function that is proficient in mass appraisal techniques, well managed, transparent, and adequately funded."[2]

Its "Evaluative Principles and Economic Concepts" section (§2.1) organizes property-tax design around four categories — administrative, social justice, economic, political — and discusses fairness, uniformity, horizontal and vertical equity, neutrality, and the regressivity debate at some length, citing A Good Tax (Youngman 2016) and Fundamentals of Tax Policy (Almy, Dornfest & Kenyon 2008) for the incidence literature.[2] Its valuation-methods section (§5.4) directs assessors to "automated mass valuation methods" and points to the Standard on Mass Appraisal of Real Property and Fundamentals of Mass Appraisal (Gloudemans & Almy 2011) for detail — the same textbook the mass-appraisal-methods page already cites as the field's methods backbone.[2]

On land value taxation, site-value taxation, split-rate taxation, or taxing land separately from improvements as a policy choice, the standard says essentially nothing. A full-text search of the document (read directly, not summarized) found land-value taxation named exactly once, in the "Additional Sources" bibliography at the back: "Dye, R., and R. England. 2010. Assessing the Theory and Practice of Land Value Taxation. Cambridge, MA: Lincoln Institute of Land Policy."[2] That citation is never discussed, quoted, or referenced anywhere in the standard's body text — it is a further-reading pointer among roughly 40 other bibliography entries, not an endorsement, critique, or analysis of site-value taxation. The wiki should not cite this standard as IAAO commentary on LVT design — as a body, IAAO's Standard on Property Tax Policy takes the conventional (land-plus-improvements, ad valorem) property tax as its frame and is silent on the split-rate/site-value alternative.

This silence is itself informative for the mass-appraisal-methods page's framing: the professional standards body whose ratio-study and mass-appraisal methodology the wiki leans on for the assessability argument has, in its policy standard, nothing to say about whether land should be taxed differently from improvements — that is a tax-base policy question the assessment-standards literature treats as out of scope, separate from the "can it be measured" question its ratio-study standard answers affirmatively.

Publication and Version Notes

• The Standard on Ratio Studies was approved April 2013 and has not been superseded by a later edition as of this reading.[1]
• The Standard on Property Tax Policy now in force is the July 2020 revision — described in its own front matter as "a complete revision" of the 2010 edition.[2] The wiki's registry previously carried this standard's year as 2010 (the edition Doucet links); this page cites and dates the current 2020 text, which is what a reader following the live IAAO URL will actually receive.
• Both documents carry IAAO's standard advisory-status caveat (added February 2022): "IAAO standards are advisory in nature and the use of, or compliance with, such standards is voluntary. If any portion of these standards is found to be in conflict with national, state, or provincial laws, such laws shall govern."[1][2]

See Also

• International Association of Assessing Officers (IAAO) — the standards body
• Mass Appraisal Methods — the concept page these standards underpin
• Objection: Land value can't be assessed accurately
• Hefferan & Boyd: Mass Appraisal Valuations in Australia — source of the 2% appeal-rate benchmark (not this standard)
• Kolbe et al.: Berlin land-value appraisal — the empirical accuracy evidence these definitions measure
• Mass Appraisal International Cases (six-country synthesis)
• Gloudemans & Almy, Fundamentals of Mass Appraisal — the textbook this standard cites page-by-page for land valuation, ratio-study statistics, and cost/sales/income modeling

Sources

1. International Association of Assessing Officers (2013), Standard on Ratio Studies, approved April 2013. PDF (read in full, 2026-07-18) — used for the definitions and formulas of COD, PRD, and PRB; the COD acceptable-range tables for vacant land vs. improved property (Table 1-3, Table 2-3); the PRD (0.98–1.03) and PRB (±0.05 general, ±0.10 unacceptable) standard ranges; the explicit land-only ratio-study instruction (§3.5); and confirmation that no 2% appeal-rate benchmark appears in this document.
2. International Association of Assessing Officers (2020), Standard on Property Tax Policy, revised and approved July 2020 (a complete revision of the 2010 edition). PDF (read in full, 2026-07-18) — used for the "core principles" quotation, the evaluative-principles/regressivity discussion, the valuation-methods cross-reference to the Standard on Mass Appraisal and Fundamentals of Mass Appraisal (Gloudemans & Almy 2011), and the confirmation that land/site-value taxation appears only once, in the bibliography (Dye & England 2010), with no body-text discussion.