Geographic Access Gravity Model

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Geographic Access Gravity Model

• Statement of Problem (Measurement)
• The Theory and Method (Potential Accessibility)
• Applications (Examples -Preliminary
  Maps)
• Future Directions (Data Model
  Improvements)
• Feedback from Others (Discussion)

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The Problem(s) - Measurement

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

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The Problem(s)

• Reliable MEASUREMENT requires a common scale that
  allows for relative comparison of values.
• Reliable MEASUREMENT requires a method to handle
  arbitrary boundaries imposed by a data collection
  geographic framework.

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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 commonly used
  geographic component and provides sufficient
  spatial resolution (many and delineated following
  population). They are our choice.

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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 (coverage)
  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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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. Problem of arbitrary boundaries.
• 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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The Theory

• Spatial Interaction - The closer together
  phenomena are the easier it is for interaction to
  take place and the more similar they are.
  (Toblers Law 1st Law of Geography).
• Spatial Interaction - Assumed to decline with
  increasing distance.
• Distance Decay - The result of declining
  interaction - termed friction of distance.

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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.

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Original (Retail) Gravity Model
or
Interaction between two areas i and j
Population of each area
Distance between areas
Distance exponent - the higher the greater the
friction of distance
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Market area definition (polygons)
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Potential AccessibilityDGRs Gravity Model
PAj Potential Accessibility for ZIP Code j
popi Population of ZIP Code i
provi Number of providers/facilitie
s in ZIP Code i   f(dij) 1
for all dij all 35 for all dij 100  
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Distance Decay(Rule-Based Function)
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Test Applications

• Have done initial development and testing.
• Preliminary survey data used to test gravity
  model.
• Also population estimates for ZIP Codes.
• Test maps for
• Hospital Beds
• Primary Care Physicians
• Registered Nurses
• General Dentists

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Future Directions

• Use new (improved) data from licensing boards on
  a quarterly basis.
• Train HPC staff to use the Excel and SAS based
  version.
• Develop with ArcView 3.x and Avenue scripts (was
  being considered in 1999).
• Develop a new version using ArcGIS (ArcMap) and
  ArcObjects (VB/A).
• Address the Edge Problem --
  Peripheral area data (Arizona, Colorado, Texas
  and Mexico?)

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Future Directions

• Enhance gravity model for demographic and risk
  factor analyses. (multivariate techniques data
  available from ESRI BIS/ArcView Business
  Analyst).
• 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
• Has MORE DETAIL Was Up to date

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