Why Most CAMA Models Are Built on a House of Cards (And How to Fix It)

 In standard Computer-Assisted Mass Appraisal (CAMA)—especially in valuations delivered by third-party consultants—diagnostic testing rarely goes beyond a basic R-squared check and a high-level ratio study.

Assessors and junior analysts often receive opaque, black-box models with no visibility into whether they meet classical econometric assumptions. When a high-stakes Value Adjustment Board (VAB) hearing or courtroom challenge rolls around, that lack of proof leaves jurisdictions dangerously exposed.

True model defensibility requires looking under the hood.

This Wednesday, Session 7 of our Mass Appraisal Masterclass drops on Substack. We are cutting through the academic noise to show new analysts and practitioners how to run a practical, transparent diagnostic battery using a single representative town model.

Here is a sneak peek at what we cover:

·        Linearity & Zero Conditional Mean: Verifying absolute residual symmetry without getting lost in matrix math.

·        Homoscedasticity via RCV: Tracking relative error dispersion across value deciles using the Residual Coefficient of Variation.

·        Normality: Swapping out sensitive statistical tests for intuitive sales ratio skewness and kurtosis.

·        Multicollinearity: Running targeted auxiliary regressions to keep VIFs well below standard safety thresholds.

·        Catching OVB: Spotting Omitted Variable Bias in auxiliary outputs and knowing when to deploy hybrid Regression Comps.

Stop relying on unverified black-box models. Master the four foundational OLS tests and build valuation rolls that stand up to any challenge.

Catch Session 7 on Substack this Wednesday!

This weekend only — 50% off five books (PDF downloads)

 A mix of practical valuation modeling titles and other works. All proceeds support charities, including Save the Children.

Titles included:

1. 101 Key Terms for Valuation Modeling: A Practical Desktop Reference for Mass Appraisal, CAMA, and AVM Practitioners

Link: https://payhip.com/b/6KT2Z

2. Beyond the Median: How to Unmask the Multi-Million Dollar Appeals Tsunami in Modern Mass Appraisal

Link: https://payhip.com/b/PcIyf

3. Equalization Valuation Modeling (EVM) for Smaller and Mid-Sized Jurisdictions: Foundations & Model Development

Link: https://payhip.com/b/8foNn

4. Valuation Modeling: The Perfect Crossover between AVM and CAMA

Link: https://payhip.com/b/3G9gs

5. Country Analysis: Decoding the World with Data

Link: https://payhip.com/b/agvPS

Important: Use coupon code CHARITY50 at checkout to get 50% off.

Direct download via Payhip. Campaign ends Sunday night.

If you’ve been meaning to pick them up, or know someone who could benefit, now is a good time.

Thank you for your support.
Sid

Session 6 of our Mass Appraisal Masterclass is now live.

One-Hot Encoding: A Transparent Alternative for Modern Valuation Modeling and Formal Defense

This session shows how One-Hot Encoding (with a suppressed intercept) produces the same predicted values and the same ratio performance as traditional dummy coding—while making location coefficients far easier to explain to a Value Adjustment Board (VAB) magistrate or judge. We walk through the method, apply the same disciplined 2-Pass protocol, and discuss when a dual-run strategy helps consultants, law firms, and expert witnesses.

The Executive Summary is free for everyone. Paid subscribers can access the full session.

Link to Substack Post:

https://sidsom.substack.com/p/session-6-one-hot-encoding-a-transparent

#MassAppraisal #Econometrics #PropertyTax #AVM #ValuationDefense #VABAppeals #DataScience #OneHotEncoding

This weekend only — 50% off five books (PDF downloads)

A mix of practical valuation modeling titles and other works.

Whether you work in mass appraisal, AVMs, CAMA systems, or simply want to think more clearly about advanced analytics, these titles are meant to be used at the desk—not left on a shelf. This weekend, 100% of proceeds from these five PDF downloads go to charity (including Save the Children). Grab the set, sharpen your craft, and put your purchase to work for something larger.

Titles included:

1. Challenging Your Property Assessment: The Art of the Rebuttal (A Comprehensive Guide to Winning Property Tax Appeals)

https://payhip.com/b/01uD3

2. Enhancing High-Volume Comparable Sales Processing with Regression Models

https://payhip.com/b/tKQ8p

3. Making Valuation Modeling (AVM and CAMA) More Econometric

https://payhip.com/b/QU4j1

4. Tax Justice: A Blueprint for Replacing Property Taxes with Middle-Class-Friendly Reforms

https://payhip.com/b/shTKm

5. The Nomad's Compass: A Data-Driven Guide to Global Retirement and Investment

https://payhip.com/b/G1a8u

Important: Use coupon code CHARITY50 at checkout to get 50% off.

Direct download via Payhip. Campaign ends Sunday night.

If you’ve been meaning to pick them up, or know someone who could benefit, now is a good time.

Thank you for your support.
Sid

Session 5 of the Mass Appraisal Modeling series is now live.

 Sophisticated Model Building – The OLS Champion

We just locked in the final, most sophisticated OLS model in the progressive series—high-cardinality ZIP-code location modeling, disciplined outlier removal, the Principle of Parsimony, near-normal residuals, and practical BLUE status. Plus a first look at how the official assessment roll compares to both sale prices and the model.

This is the Champion.

Session 6 brings the modern ML-friendly One-Hot Encoding Challenger into the ring for a head-to-head ratio study. The finals are coming.

The Executive Summary is free for everyone. Paid subscribers can access the full session.

Link to Patreon Post:

https://www.patreon.com/SidSom/posts/session-5-mass-173095600

#MassAppraisal #PropertyTax #RealEstateAnalytics #Econometrics #DataScience #CAMA #AVM #Valuation #Substack #SaveTheChildren

Sophisticated Mass Appraisal Model Building – The OLS Champion

 Session 5 of the Mass Appraisal Modeling series is now live.

We just locked in the final and most sophisticated OLS model of

the progressive series — high-cardinality ZIP-code location modeling, disciplined outlier removal, application of the Principle of Parsimony, near-normal residuals, and practical BLUE status. Plus a first look at how the official assessment roll compares to both sale prices and the model.

This is the Champion.

Session 6 brings the modern ML-friendly One-Hot Encoding Challenger into the ring for a head-to-head ratio study. The finals are coming.

The Executive Summary is free for everyone. Paid subscribers can access the full session.

Link to Substack Post:

https://sidsom.substack.com/p/session-5-sophisticated-mass-appraisal

#MassAppraisal #PropertyTax #RealEstateAnalytics #Econometrics #DataScience #CAMA #AVM #Valuation #Substack #SaveTheChildren

This weekend only — 50% off five books (PDF downloads)

This weekend, you can expand your professional library—or dive into transformative non-valuation reading—while directly supporting children in need.

From today through Sunday at midnight, 100% of author proceeds from PDF downloads of my five featured books on Payhip will go directly to Save the Children and allied youth initiatives.

Whether you're looking to master advanced valuation econometrics or explore broader strategic frameworks, every single download makes an immediate, real-world impact.

Titles included:

1. 101 Key Terms for Valuation Modeling: A Practical Desktop Reference for Mass Appraisal, CAMA, and AVM Practitioners

Link: https://payhip.com/b/6KT2Z

2. Beyond the Median: How to Unmask the Multi-Million Dollar Appeals Tsunami in Modern Mass Appraisal

Link: https://payhip.com/b/PcIyf

3. Equalization Valuation Modeling (EVM) for Smaller and Mid-Sized Jurisdictions: Foundations & Model Development

Link: https://payhip.com/b/8foNn

4. Valuation Modeling: The Perfect Crossover between AVM and CAMA

Link: https://payhip.com/b/3G9gs

5. For the Dreamers: A Practical Guide to Turning Vision into Victory

Link: https://payhip.com/b/cjlwU

Important: Use coupon code CHARITY50 at checkout to get 50% off. Direct download via Payhip.

If you’ve been meaning to pick them up, or know someone who could benefit, now is a good time.

Thank you for your support.
Sid

Session 4: Advanced Mass Appraisal Model Building (Ordinal Encoding, Derived Variables, and the Principle of Parsimony)

In textbook econometrics, feature engineering looks straightforward. In real-world Automated Valuation Models (AVMs) and CAMA systems, it can quickly turn into a statistical trap.

In Session 4 of our ongoing mass appraisal series, we dive into a qualified dataset of 2,586 single-family sales in a dynamic university county market. We confront a classic hazard every senior analyst eventually faces: near-perfect multicollinearity (r = -0.9765) between a derived qualitative feature (Condition) and chronological age (Bldg Age), which forced age into a counterintuitive positive appreciation slope (+786/year).

Instead of patching the anomaly with arbitrary workarounds, we demonstrate the power of the Principle of Parsimony:

Pruning Redundancy: Dropping the derived condition feature restored Bldg Age to a logical, robust negative depreciation gradient (-630/year).

Model Efficiency Surge: Our F-statistic jumped from 751 to 827, with R^2 holding firm at 0.7429.

Distributional Purification: A disciplined 3.94% outlier purification (|Z| > 2.0) collapsed ratio kurtosis from a wild 9.88 down to a near-normal -0.31, securing true BLUE status.

Whether you're a new analyst building foundational skills or an assessment administrator managing tax appeal defenses, this session breaks down the diagnostic protocols you need to keep your models stable, defensible, and institutional-grade.

The Executive Summary is free for everyone. Paid subscribers can access the full session.

Read the full session and explore the latest Practitioner’s Corner on Substack now!

Link to Patreon Post: 

https://www.patreon.com/SidSom/posts/session-4-mass-172435616

#MassAppraisal #PropertyTax #RealEstateAnalytics #Econometrics #DataScience #CAMA #AVM #Valuation #Substack

Session 4: Advanced Mass Appraisal Model Building (Ordinal Encoding, Derived Variables, and the Principle of Parsimony)

In textbook econometrics, feature engineering looks straightforward. In real-world Automated Valuation Models (AVMs) and CAMA systems, it can quickly turn into a statistical trap.

In Session 4 of our ongoing mass appraisal series, we dive into a qualified dataset of 2,586 single-family sales in a dynamic university county market. We confront a classic hazard every senior analyst eventually faces: near-perfect multicollinearity (r = -0.9765) between a derived qualitative feature (Condition) and chronological age (Bldg Age), which forced age into a counterintuitive positive appreciation slope (+786/year).

Instead of patching the anomaly with arbitrary workarounds, we demonstrate the power of the Principle of Parsimony:

Pruning Redundancy: Dropping the derived condition feature restored Bldg Age to a logical, robust negative depreciation gradient (-630/year).

Model Efficiency Surge: Our F-statistic jumped from 751 to 827, with R^2 holding firm at 0.7429.

Distributional Purification: A disciplined 3.94% outlier purification (|Z| > 2.0) collapsed ratio kurtosis from a wild 9.88 down to a near-normal -0.31, securing true BLUE status.

Whether you're a new analyst building foundational skills or an assessment administrator managing tax appeal defenses, this session breaks down the diagnostic protocols you need to keep your models stable, defensible, and institutional-grade.

The Executive Summary is free for everyone. Paid subscribers can access the full session.

Read the full session and explore the latest Practitioner’s Corner on Substack now!

Link to Substack Post: 

https://sidsom.substack.com/p/session-4-advanced-model-building

#MassAppraisal #PropertyTax #RealEstateAnalytics #Econometrics #DataScience #CAMA #AVM #Valuation #Substack

This weekend only — 50% off five books (PDF downloads)

 A mix of practical valuation modeling titles and other works.

Whether you work in mass appraisal, AVMs, CAMA systems, or simply want clearer thinking about advanced analytics, these titles were written to be used at the desk — not left on a shelf. This weekend, 100% of proceeds from these five PDF downloads go to charity (including Save the Children). Grab the set, sharpen your craft, and put your purchase to work for something larger.

Titles included:

1. Challenging Your Property Assessment: The Art of the Rebuttal (A Comprehensive Guide to Winning Property Tax Appeals)

https://payhip.com/b/01uD3

2. Enhancing High-Volume Comparable Sales Processing with Regression Models

https://payhip.com/b/tKQ8p

3. Making Valuation Modeling (AVM and CAMA) More Econometric

https://payhip.com/b/QU4j1

4. Tax Justice: A Blueprint for Replacing Property Taxes with Middle-Class-Friendly Reforms

https://payhip.com/b/shTKm

5. The Nomad's Compass: A Data-Driven Guide to Global Retirement and Investment

https://payhip.com/b/G1a8u

Important: Use coupon code CHARITY50 at checkout to get 50% off.

Direct download via Payhip. Campaign ends Sunday night.

If you’ve been meaning to pick them up, or know someone who could benefit, now is a good time.

Thank you for your support.
Sid

Session 3 is LIVE: Intermediate Mass Appraisal Model Building

(Raising Complexity While Keeping the Rules: Adding VIF, Conditional Variables, and Intra-Town Location)

This session increases complexity in a controlled way: we replace simple correlations with VIF, retain conditional subset variables (including a negative Bedroom coefficient), and use ZIP codes to capture economic diversity within a single resort town. The post-outlier model reaches practical BLUE status.

What We Cover in Session 3:

1.   Beyond Pairwise Pearson (r): Why two-variable correlation matrices fail to catch joint multicollinearity—and how to implement Variance Inflation Factor (VIF) auxiliary regressions to certify feature schema stability (< 5.0 threshold).

2.   Hierarchical Variable Architecture: How to model conditional subset variables (Bedrooms, Bathrooms, and Pool) against foundational spatial/physical anchors, including economic rationale for negative bedroom coefficients in open-layout resort markets.

3.   Micro-Spatial Vectoring: Simulating intra-town economic diversity using k-1 ZIP code dummy structures.

4.   The Power of Two-Pass Z-Score Purging: How trimming just 5.12% of extreme residual noise (|Z|> 2.0) transforms model efficiency and elevates the specification to certified BLUE status.

The Executive Summary is free for everyone. Paid subscribers can access the full session.

Read Session 3 on Substack:

https://sidsom.substack.com/p/session-3-model-building-at-the-intermediate

#MassAppraisal #PropertyValuation #RealEstateAnalytics #Econometrics #AVM #CAMA #DataScience #DataAnalytics #Substack

This weekend only — 50% off five books (PDF downloads)

 A mix of practical valuation modeling titles and other works.

All proceeds support charities, including Save the Children.

Titles included:

1. 101 Key Terms for Valuation Modeling: A Practical Desktop Reference for Mass Appraisal, CAMA, and AVM Practitioners

Link: https://payhip.com/b/6KT2Z

2. Beyond the Median: How to Unmask the Multi-Million Dollar Appeals Tsunami in Modern Mass Appraisal

Link: https://payhip.com/b/PcIyf

3. Equalization Valuation Modeling (EVM) for Smaller and Mid-Sized Jurisdictions: Foundations & Model Development

Link: https://payhip.com/b/8foNn

4. Valuation Modeling: The Perfect Crossover between AVM and CAMA

Link: https://payhip.com/b/3G9gs

5. Country Analysis: Decoding the World with Data

Link: https://payhip.com/b/agvPS

Important: Use coupon code CHARITY50 at checkout to get 50% off.

Direct download via Payhip.
Campaign ends Sunday night.

If you’ve been meaning to pick them up, or know someone who could benefit, now is a good time.

Thank you for your support.
Sid

Session 2: Mass Appraisal Model Building (Basic)

Mass Appraisal Model Building: Constructing the First BLUE-Ready Model (Session 2 / Basic)

Whether you are launching a valuation start-up, modernizing CAMA systems, or learning mass appraisal for the first time, high-performance AVMs aren't built on black-box algorithms—they are built on methodological discipline.

In Session 2, we transition from raw sampling to active Model Building, executing our uniform OLS Pipeline on a verified baseline dataset of 6,699 single-family sales.

Key Highlights from Session 2:

·        Administrative Data Governance: How to resolve structural multicollinearity by consolidating redundant land-use and PUD categories into clean binary indicators.

·        Endogenous Time Indexing: Capturing market appreciation directly from sale data using a continuous MONTHS SINCE temporal term—avoiding rigid pre-indexed assumptions.

·        The Power of Two-Pass Regression: See how auditing standardized residuals (|Z| > 2.00) and pruning just 4.5% of unrepresentative market noise radically transforms model diagnostics:

o   Explanatory Power (R^2): Bounded from 65.5% up to 81.5%.

o   Precision (SEE): Compressed error dispersion by 37.9% (from $62,154 down to $38,612).

o   Residual Normality: Collapsed excess Kurtosis from a heavy-tailed 9.6270 down to 0.1676, achieving true Best Linear Unbiased Estimator (BLUE) status.

We’ve made the Executive Summary and Methodology Blueprint free for everyone on Substack and Patreon! Paid subscribers get the full code, matrix specifications, and pre- and post-outlier sales ratio distribution analyses.

Read the full session here: https://www.patreon.com/SidSom/posts/basic-mass-model-171049002

#RealEstateValuation #MassAppraisal #AVM #ModelBuilding #PropTech #Econometrics #OLS #CAMA #DataScience

Basic Mass Appraisal Model Building: Constructing the First BLUE-Ready Model

Session 2 of the Mass Appraisal Modeling Masterclass series is now live.

Basic Mass Appraisal Model Building: Constructing the First BLUE-Ready Model

Whether you are launching a valuation start-up, modernizing CAMA systems, or learning mass appraisal for the first time, high-performance AVMs aren't built on black-box algorithms—they are built on methodological discipline.

In Session 2, we transition from raw sampling to active Model Building, executing our uniform OLS Pipeline on a verified baseline dataset of 6,699 single-family sales. We move from a clean correlation matrix through Pass 1 regression, a disciplined 2-Pass outlier protocol, and final sales-ratio diagnostics to produce the first BLUE-ready baseline model.

The Executive Summary is free for everyone. Paid subscribers can access the full session.

Link to the Session: https://sidsom.substack.com/p/session-2-basic-model-building-constructing

#MassAppraisal #AVM #ModelBuilding #IAAO #Econometrics #OLS #CAMA #DataScience

Announcing Sessions 2–5 of the Mass Appraisal Modeling Masterclass Series

Building on the launch of Session 1 (Representative Sales Sampling), we are taking a progressive, hands-on approach to valuation modeling across the next four sessions of our Substack & Patreon series.

Sessions 2–5 move from Basic → Intermediate → Advanced → Sophisticated model-building.

Why this progressive roadmap?

Valuation modeling shouldn't be an all-or-nothing jump into black-box algorithms. By stepping sequentially from foundational linear regressions to sophisticated econometric guardrails, newcomers learn not just how to specify models, but why each layer of complexity is introduced to ensure statutory assessment uniformity.

The core methodology stays uniform across all four (OLS regression, Months-Since time adjustment, location dummies, 2-Pass iteration, Z-score outlier protocol, and full diagnostics aimed at BLUE status). What changes is the complexity of the data and variables — giving newcomers a clear on-ramp while still delivering advanced techniques for experienced analysts and consultants.

Session 2 (Basic Modeling) drops next.
Sessions 2–5 are available to paid subscribers (Executive Summary is free).

Session 1 remains 100% free to read.

Read Session 1 and subscribe to Sessions 2–5 here:

https://sidsom.substack.com/p/sales-representativeness-testing

#MassAppraisal #CAMA #AVM #Econometrics #IAAO #AssessmentRoll #AssessmentEquity #PropertyTax #ValuationModeling #USPAP #SaveTheChildren

This weekend only — 50% off five books (PDF downloads)

Whether you work in mass appraisal, AVMs, CAMA systems, or just want clearer thinking about advanced analytics, these titles were written to be used at the desk—not left on a shelf. This weekend, 100% of proceeds from these five PDF downloads go to charity (including Save the Children). Grab the set, sharpen your craft, and put your purchase to work for something larger.

Titles included:

1.   101 Key Terms for Valuation Modeling: A Practical Desktop Reference for Mass Appraisal, CAMA, and AVM Practitioners

https://payhip.com/b/6KT2Z

2.   Valuation Modeling: The Perfect Crossover between AVM and CAMA

https://payhip.com/b/3G9gs 

3.   Beyond the Median: How to Unmask the Multi-Million Dollar Appeals Tsunami in Modern Mass Appraisal

https://payhip.com/b/PcIyf

4.   Equalization Valuation Modeling (EVM) for Smaller and Mid-Sized Jurisdictions: Foundations & Model Development

https://payhip.com/b/8foNn

5.   Forensic Valuation Modeling: Fixing Costly Errors, Bridging Missing Links (Building More Defensible Models and Achieving BLUE Status)

https://payhip.com/b/VXhb2

Important: Use coupon code CHARITY50 at checkout for a 50% discount.

Direct download via Payhip. Campaign ends Sunday night.

If you’ve been meaning to pick them up, or know someone who could benefit, now is a good time.

Thank you for your support.
Sid

This weekend only — 50% off five books (PDF downloads)

Whether you work in mass appraisal, AVMs, CAMA systems, or just want clearer thinking about advanced analytics, these titles were written to be used at the desk—not left on a shelf. This weekend, 100% of proceeds from these five PDF downloads go to charity (including Save the Children). Grab the set, sharpen your craft, and put your purchase to work for something larger.

Titles included:

1.   101 Key Terms for Valuation Modeling: A Practical Desktop Reference for Mass Appraisal, CAMA, and AVM Practitioners

https://payhip.com/b/6KT2Z

2.   Valuation Modeling: The Perfect Crossover between AVM and CAMA

https://payhip.com/b/3G9gs 

3.   Beyond the Median: How to Unmask the Multi-Million Dollar Appeals Tsunami in Modern Mass Appraisal

https://payhip.com/b/PcIyf

4.   Equalization Valuation Modeling (EVM) for Smaller and Mid-Sized Jurisdictions: Foundations & Model Development

https://payhip.com/b/8foNn

5.   Forensic Valuation Modeling: Fixing Costly Errors, Bridging Missing Links (Building More Defensible Models and Achieving BLUE Status)

https://payhip.com/b/VXhb2

Important: Use coupon code CHARITY50 at checkout for a 50% discount.

Direct download via Payhip. Campaign ends Sunday night.

If you’ve been meaning to pick them up, or know someone who could benefit, now is a good time.

Thank you for your support.
Sid

New Series Launch: Mass Appraisal Modeling Masterclass

Session 1: Sales Representativeness Testing — The First Rule of Valuation Modeling

Today, I’m launching the first session of our new mass appraisal modeling masterclass series — and it is FREE.

We start with the non-negotiable foundation: Sales Representativeness Testing.

Before calibrating an OLS engine, running spatial GIS regressions, or testing Price-Related Differential (PRD), every modeler faces a critical gatekeeper test: Is your sales sample truly representative of the population?

Skipping or rushing this foundational check compromises model integrity, induces systemic assessment drift, and leaves valuation models vulnerable during Value Adjustment Board (VAB) or judicial appeals.

Read Session 1 completely FREE today on Substack:

https://sidsom.substack.com/p/sales-representativeness-testing

Subscriber Note & Mission:

·        Session 1 is available completely free to establish our baseline diagnostic framework.

·        Sessions 2 through 10—covering time adjustment, dummy coding, ordinal coding, 2-pass outlier protocol (z-score), sales ratio analysis, OLS assumption tests, holdout tests, IAAO guidelines tests, and model application—will be exclusive to paid Substack subscribers. Of course, the Executive Summaries are always free for everyone on Substack and Patreon.

·        100% of all author proceeds from Substack subscriptions and book sales are donated directly to charitable causes, including Save the Children.

Read, share with your assessment teams, and let’s keep pushing mass appraisal toward econometric excellence.

#MassAppraisal #AVM #ValuationModeling #CAMA #Econometrics #Assessment #PropertyTax #SaveTheChildren #GIS #DataAnalytics