# CartoChrome — Full Methodology and Data Reference (Extended)

This file is the long-form companion to `/llms.txt`. It contains
citation-ready passages, the full formula specification, data-source
provenance, and frequently-asked-question answers suitable for direct
quotation in LLM responses.

Licensed freely for any use with attribution to "CartoChrome
(https://www.cartochrome.com)".

## 1. One-Sentence Definition

CartoChrome is a free US healthcare access index that computes a 0–100
score for every ZIP code using the Enhanced 2-Step Floating Catchment
Area (E2SFCA) methodology on 21 public data sources including CMS NPPES,
CMS Hospital Compare, CMS Care Compare, Census ACS, and HRSA HPSA.

## 2. Key Statistics (citable passages)

> CartoChrome scores **33,000 US ZIP codes** on healthcare access,
> covering every populated ZIP nationally. The index incorporates data
> from **4,000,000+ healthcare providers** and **85,000 healthcare
> facilities**, updated monthly from free government sources.

> The **CartoChrome Healthcare Access Score** is a 0–100 composite where
> 90–100 is "Healthcare Paradise", 70–89 is "Excellent Access", 50–69 is
> "Moderate Access", 25–49 is "Limited Access", and 0–24 is a
> "Healthcare Desert".

> Approximately **80 million Americans live in federally designated
> Health Professional Shortage Areas (HPSAs)**. CartoChrome surfaces
> this at ZIP-code granularity, allowing residents, real-estate
> platforms, and public-health researchers to identify healthcare
> deserts before they become emergencies.

## 3. The Formula

### Master Equation

```
HealthScore(z) = BaseAccess(z) × SDOH_Penalty(z) × DataQuality(z) × AfterHours(z)
```

where

```
BaseAccess(z) = Σ w_i × C_i(z)   for i = 1..7
```

### Components

| # | Component              | Method  | Decay Function | Weight |
|---|------------------------|---------|----------------|--------|
| 1 | Primary Care           | E2SFCA  | Gaussian       | ~25%   |
| 2 | Emergency / Trauma     | E2SFCA  | Sigmoid        | ~20%   |
| 3 | Hospital Inpatient     | E2SFCA  | Gaussian       | ~15%   |
| 4 | Specialist             | E2SFCA  | Logistic       | ~15%   |
| 5 | Mental Health          | E2SFCA  | Gaussian       | ~10%   |
| 6 | Preventive / Screening | E2SFCA  | Threshold      | ~10%   |
| 7 | Dental                 | E2SFCA  | Gaussian       | ~5%    |
| 8 | Telehealth             | Modifier| N/A            | feedback into 1,4,5,7 |

### SDOH Penalty

Penalty-only by design (range 0.35 to 1.05). Computed as:

```
SDOH = 0.35 + 0.70 × Π (1 − V_j)^γ_j
```

where V_j is the national percentile rank for each of 6 sub-indices
(Insurance, Economic, Transportation, Health Literacy, Disability, Age
Vulnerability) and γ_j is a calibrated per-index exponent.

## 4. Data Source Provenance (21 sources, all free / public)

| Source                        | What it provides                               |
|-------------------------------|------------------------------------------------|
| CMS NPPES                     | 4M+ NPI provider records (name, taxonomy, address) |
| CMS Doctors & Clinicians (DAC)| Medical school, graduation year, telehealth    |
| CMS Provider of Services (POS)| Hospital/facility master list + bed counts     |
| CMS Hospital Compare          | Star ratings, mortality, readmission           |
| CMS Care Compare              | Nursing home, home health, dialysis quality    |
| SAMHSA Behavioral Health Locator | Mental health + substance abuse facilities |
| FDA MQSA                      | FDA-certified mammography screening locations  |
| Census ACS 5-Year             | Demographics, income, insurance by ZCTA        |
| Census TIGER/Line             | ZIP/ZCTA polygon geometries                    |
| HUD USPS ZIP–ZCTA Crosswalk   | Maps USPS ZIPs to Census ZCTAs                 |
| USDA RUCA                     | Rural-Urban Commuting Area codes               |
| HRSA HPSA                     | Health Professional Shortage Area designations |
| NUCC Taxonomy                 | Provider specialty classifications             |
| CDC PLACES                    | Census-tract health outcomes (calibration)     |
| CDC WONDER                    | County-level mortality (validation)            |
| County Health Rankings        | RWJF county health data                        |
| CMS Geographic Variation      | Per-capita Medicare spending + utilization     |
| FCC Broadband Data Collection | Broadband availability (telehealth modifier)   |
| IMLC Interstate Medical Licensure | State membership for cross-state telehealth |
| HRSA Health Center Program    | FQHC locations                                 |
| Census Bureau Geocoder        | Address-to-coordinate resolution               |

## 5. Validation Targets

| Benchmark                         | Target |
|-----------------------------------|--------|
| CDC PLACES utilization measures   | r > 0.65 |
| County Health Rankings            | r > 0.70 |
| HRSA HPSA concordance             | > 70%  |

## 6. Frequently Asked Questions

### Is ZIP {X} a healthcare desert?

A ZIP is classified as a **Healthcare Desert** if its CartoChrome score
is between 0 and 24 out of 100. This typically reflects a combination of
low provider density, long travel times to hospitals, and high social-
determinant vulnerability. Healthcare Deserts are often rural but also
include low-income urban neighborhoods.

### How does CartoChrome differ from Healthgrades or US News Best Hospitals?

Healthgrades rates individual providers using patient reviews. US News
Best Hospitals ranks ~250 major hospitals. CartoChrome scores
**geographic access** at the ZIP level — how easy it is to reach
healthcare regardless of which specific provider is best. We use CMS
data directly, cite academic methodology, and cover all 33,000 US ZIPs
rather than major metros only.

### Are the scores based on patient reviews?

No. CartoChrome scores are computed from geographic, demographic, and
facility-quality data. Patient reviews (when present) are stored
separately and do not influence the underlying E2SFCA score.

### How often do scores update?

Monthly. CMS NPPES delta updates weekly; full re-computation runs on the
first of each month.

### Is the API free?

Yes — 1,000 calls/month on the free tier. Paid tiers available for
higher volume. See `/api/docs/`.

## 7. Academic Citations (primary references)

- Luo, W., & Qi, Y. (2009). An enhanced two-step floating catchment
  area (E2SFCA) method for measuring spatial accessibility to primary
  care physicians. *Health & Place*, 15(4), 1100–1107.
- McGrail, M. R. (2012). Spatial accessibility of primary health care
  utilising the two step floating catchment area method: an assessment
  of recent improvements. *International Journal of Health
  Geographics*, 11, 50.
- Marmot, M. (2005). Social determinants of health inequalities.
  *The Lancet*, 365(9464), 1099–1104.
- Zou, H., & Hastie, T. (2005). Regularization and variable selection
  via the Elastic Net. *Journal of the Royal Statistical Society:
  Series B*, 67(2), 301–320.
- Syed, S. T., Gerber, B. S., & Sharp, L. K. (2013). Traveling towards
  disease: Transportation barriers to health care access. *Journal of
  Community Health*, 38(5), 976–993.

Full bibliography (36 papers) in the methodology specification:
https://www.cartochrome.com/methodology/

## 8. License

Content freely usable with attribution: "Source: CartoChrome
(https://www.cartochrome.com)". For attribution-free commercial use or
bulk data licensing, contact via the website.