Geographic Access Gravity Model

1. Statement of Problem (Measurement)

2. The Theory and Method (Potential Accessibility)

3. Applications (Preliminary Maps)

4. Future Directions (Data & Model Improvements)

5. Feedback from Others (Discussion)

1

The Problem(s) - Measurement

• How to MEASURE geographic access to health care providers and facilities?

• The Solution - Develop a reliable method to MEASURE (and compare) the distribution of facilities/providers and the population.

• Reliable MEASUREMENT requires a geographic framework in which to collect and organize observations.

2

The Problem(s)

• Reliable MEASUREMENT requires a common scale that allows for comparison of values.

• Reliable MEASUREMENT requires a method to handle arbitrary boundaries imposed by a data collection geographic framework.

3

The Geographic Framework - ZIP Codes

• Health care data (patient, provider, facility, etc.) have many geographic (locational) components.

• Some geographic components are; geographic coordinates, county, census block and tract, etc.).

• An address with a ZIP Code is a widely used geographic component (our choice).

4

NM ZIP Codes

• US Postal Service delivery areas.• Some ZIP Codes do not have rural

delivery and mail is picked up at the Post Office.

• There can be multiple ZIP Codes per post office (delivery and no delivery).

• DGR has prepared a ZIP Code base map for NM with 400 ZIP Codes for mapping HPC Data (new ZIP Codes are being added - Currently 403).

• We added (estimated) boundaries for 125 ZIP Codes (USPS review).

5

DGR’s ZIP Code Maps

1999 and 2002

Map of ZIP Codes Added and Estimated

A Common Measurement Scale

• Service Capacity Standards (traditional measure - Fed. and State guidelines).

• Ratio of provider or facilities per population.

• Can be expressed as either:

• One M.D. per 1,500 persons (Prov./ Pop.)

• 1,500 persons per M.D. (POP. / Prov.) **What we use.

8

The Boundary Problem

• Traditional measures (service capacity standards) NOT very good:

• Does NOT consider that people move among communities or political/data collection units to obtain medical services.

• Nobody pays attention to what ZIP Code their doctor is in.

• Does NOT consider distance (close or far).

9

Map of Dentists per ZIP Code

The Theory

• Spatial Interaction - The closer together phenomena are the easier it is for interaction to take place.

• Spatial Interaction - Assumed to decline with increasing distance.

• Distance Decay - The result of declining interaction - termed friction of distance.

11

The Method (Spatial Analysis)

• Gravity Models have been used in economics and social sciences since William Reily (Univ. of Texas) proposed the idea in 1929.

• Gravity Models have been traditionally used in retail studies, but recently in health care.

• Gravity Models allow for the measurement of spatial interaction as a function of distance.

12

Original (Retail) Gravity Model

ij

jiij

dPPI b ijjiij dPPI b

or

ijI Interaction between two areas i and j

ji PP and Population of each area

ijd Distance between areas

b Distance exponent - the higher the greater the friction of distance

** Market area definition (polygons)13

Potential AccessibilityDGR’s Gravity Model

n

i

iij

n

i

iij

j

provdf

popdfPA

1

1

Code for ZIPity Accessibil PotentialPAj

Code ZIPa of Population ipop

Code in ZIP FacilitiesProviders/ ofNumber iprov

100 d if 0

100 d 35 if 235 d if 1

ddij 14

Applications

• Still doing initial development and testing.

• Preliminary survey data used to test gravity model.

• Test maps for:– Hospital Beds– Primary Care Physicians– Registered Nurses– General Dentists

15

Applications

• New data available shortly from licensing boards.

• Demographic data being evaluated.

• Risk factor (population segments at risk for receiving inadequate care) applications based on demographic data are being considered.

16

Preliminary (Test) Gravity Model Maps

Future Directions

• Use new data from licensing boards on a quarterly basis.

• Enhance gravity model for demographic and risk factor analyses.

• Train HPC staff to use the Excel and SAS based versions along with ArcView 3.x and Avenue scripts.

• New version using ArcGIS (ArcMap) and ArcObjects (VB/A).

19

Future Directions

• Address the “Edge Problem”:– Peripheral area data (Arizona,

Colorado, Texas and Mexico ??)

• Investigate using travel time instead of straight line distance (NM roads Arc/Info network coverage)

• Review recent literature.• Prepare a presentation and

publication.20

Questions and Open Discussion

21

Internal Web Page

wwwdgr.unm.edu/hpc/hpc_grav.html

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