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).
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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.
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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).
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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
P
iPj
Iij  b
dij
or
Iij  PiPj  dij
b
Iij  Interaction between two areas i and j
Pi and Pj  Population of each area
dij 
b
Distance between areas
Distance exponent - the higher the greater
the friction of distance
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** Market area definition (polygons)
Potential Accessibility
DGR’s Gravity Model
n
 f d  pop
ij
PAj 
i
i 1
n
 f d  prov
ij
i
i 1
PAj  Potential Accessibil ity for ZIP Code
popi  Population of a ZIP Code
provi  Number of Providers/ Facilities in ZIP Code
 
1 if
d  35
dij  d  2 if 35  d   100
0 if d  100
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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.
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Questions and Open
Discussion
Internal Web Page
wwwdgr.unm.edu/hpc/hpc_
grav.html
MORE DETAIL - Up to
date
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