Geographic Access Gravity Model 1.Statement of Problem (Measurement) 2.The Theory and Method (Potential Accessibility) 3.Applications (Preliminary Maps)

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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