Geographically weighted regression

					Geographically
weighted regression

        Danlin Yu
   Yehua Dennis Wei
  Dept. of Geog., UWM
        Outline of the
        presentation
1. Spatial non-stationarity: an example
2. GWR – some definitions
3. 6 good reasons using GWR
4. Calibration and tests of GWR
5. An example: housing hedonic model
   in Milwaukee
6. Further information
1. Stationary v.s non-
      stationary
    yi= 0 + 1x1i       yi= i0 + i1x1i


   e1                      e1
                   e2                      e2

  Stationary process     Non-stationary process


  e3          e4          e3             e4


         Assumed            More realistic
                 Simpson’s paradox
              Spatially aggregated data   Spatially disaggregated data
House Price




                  House density               House density
      Stationary v.s. non-
           stationary
   If non-stationarity is modeled by
    stationary models
    – Possible wrong conclusions might be
      drawn
    – Residuals of the model might be highly
      spatial autocorrelated
    Why do relationships
      vary spatially?
   Sampling variation
    – Nuisance variation, not real spatial non-
      stationarity
   Relationships intrinsically different across
    space
    – Real spatial non-stationarity
   Model misspecification
    – Can significant local variations be removed?
    2. Some definitions
 Spatial non-stationarity: the same
  stimulus provokes a different response
  in different parts of the study region
 Global models: statements about
  processes which are assumed to be
  stationary and as such are location
  independent
        Some definitions
   Local models: spatial decompositions
    of global models, the results of local
    models are location dependent – a
    characteristic we usually anticipate
    from geographic (spatial) data
              Regression
   Regression establishes relationship among
    a dependent variable and a set of
    independent variable(s)
   A typical linear regression model looks like:
   yi=0 + 1x1i+ 2x2i+……+ nxni+i
   With yi the dependent variable, xji (j from 1
    to n) the set of independent variables, and i
    the residual, all at location i
             Regression
   When applied to spatial data, as can
    be seen, it assumes a stationary
    spatial process
    – The same stimulus provokes the same
      response in all parts of the study region
    – Highly untenable for spatial process
      Geographically
    weighted regression
 Local statistical technique to analyze
  spatial variations in relationships
 Spatial non-stationarity is assumed
  and will be tested
 Based on the “First Law of
  Geography”: everything is related with
  everything else, but closer things are
  more related
                       GWR
   Addresses the non-stationarity directly
    – Allows the relationships to vary over space, i.e.,
      s do not need to be everywhere the same
    – This is the essence of GWR, in the linear form:
    – yi=i0 + i1x1i+ i2x2i+……+ inxni+i
    – Instead of remaining the same everywhere, s
      now vary in terms of locations (i)
 3. 6 good reasons why
       using GWR
1. GWR is part of a growing trend in
   GIS towards local analysis
  •   Local statistics are spatial
      disaggregations of global ones
  •   Local analysis intends to understand the
      spatial data in more detail
         Global v.s. local
            statistics
   Global statistics           Local statistics
    – Similarity across          – Difference across
      space                        space
    – Single-valued statistics   – Multi-valued statistics
    – Not mappable               – Mappable
    – GIS “unfriendly”           – GIS “friendly”
    – Search for regularities    – Search for exceptions
    – aspatial                   – spatial
      6 good reasons why
          using GWR
2. Provides useful link to GIS
  •    GISs are very useful for the storage,
       manipulation and display of spatial data
  •    Analytical functions are not fully developed
  •    In some cases the link between GIS and
       spatial analysis has been a step backwards
  •    Better spatial analytical tools are called for to
       take advantage of GIS’s functions
         GWR and GIS
 An important catalyst for the better
  integration of GIS and spatial analysis
  has been the development of local
  spatial statistical techniques
 GWR is among the recently new
  developments of local spatial analytical
  techniques
      6 good reasons why
          using GWR
3. GWR is widely applicable to almost
   any form of spatial data
  •    Spatial link between “health” and
       “wealth”
  •    Presence/absence of a disease
  •    Determinants of house values
  •    Regional development mechanisms
  •    Remote sensing
      6 good reasons why
          using GWR
4. GWR is truly a spatial technique
  •    It uses geographic information as well
       as attribute information
  •    It employs a spatial weighting function
       with the assumption that near places
       are more similar than distant ones
       (geography matters)
  •    The outputs are location specific hence
       mappable for further analysis
      6 good reasons why
          using GWR
5. Residuals from GWR are generally
   much lower and usually much less
   spatially dependent
  •    GWR models give much better fits to
       data, EVEN accounting for added model
       complexity and number of parameters
       (decrease in degrees of freedom)
  •    GWR residuals are usually much less
       spatially dependent
Moran's I = 0.144                             Moran's I = 0.372




                                 ±


                    GWR Residuals                                 OLS Residuals
                      -.76 - -.35                                    -1.34 - -.53
                      -.34 - -.09                                    -.52 - -.19
                      -.08 - .09                                     -.18 - .08
                      .10 - .26                                      .09 - .37
                      .27 - .56                                      .38 - .92

                    0   50 100      200   300
                                            Kilometers
      6 good reasons why
          using GWR
6. GWR as a “spatial microscope”
  •    Instead of determining an optimal
       bandwidth (nearest neighbors), they can
       be input a priori
  •    A series of bandwidths can be selected
       and the resulting parameter surface
       examined at different levels of
       smoothing (adjusting amplifying factor in
       a microscope)
      6 good reasons why
          using GWR
6. GWR as a “spatial microscope”
  •    Different details will exhibit different
       spatial varying patterns, which enables
       the researchers to be more flexible in
       discovering interesting spatial patterns,
       examining theories, and determining
       further steps
    4. Calibration of GWR
   Local weighted least squares
    – Weights are attached with locations
    – Based on the “First Law of Geography”:
      everything is related with everything else,
      but closer things are more related than
      remote ones
     Weighting schemes
   Determines weights
    – Most schemes tend to be Gaussian or
      Gaussian-like reflecting the type of
      dependency found in most spatial
      processes
    – It can be either Fixed or Adaptive
    – Both schemes based on Gaussian or
      Gaussian-like functions are implemented
      in GWR3.0 and R
Fixed weighting
    scheme
        Weighting function




       Bandwidth
        Problems of fixed
            schemes
   Might produce large estimate variances
    where data are sparse, while mask subtle
    local variations where data are dense
   In extreme condition, fixed schemes might
    not be able to calibrate in local areas where
    data are too sparse to satisfy the calibration
    requirements (observations must be more
    than parameters)
Adaptive weighting
    schemes
         Weighting function




        Bandwidth
     Adaptive weighting
         schemes
   Adaptive schemes adjust itself
    according to the density of data
    – Shorter bandwidths where data are dense
      and longer where sparse
    – Finding nearest neighbors are one of the
      often used approaches
              Calibration
   Surprisingly, the results of GWR appear to
    be relatively insensitive to the choice of
    weighting functions as long as it is a
    continuous distance-based function
    (Gaussian or Gaussian-like functions)
   Whichever weighting function is used,
    however the result will be sensitive to the
    bandwidth(s)
              Calibration
   An optimal bandwidth (or nearest
    neighbors) satisfies either
    – Least cross-validation (CV) score
        CV score: the difference between observed
        value and the GWR calibrated value using the
        bandwidth or nearest neighbors
    – Least Akaike Information Criterion (AIC)
        Aninformation criterion, considers the added
        complexity of GWR models
                   Tests
   Are GWR really better than OLS
    models?
    – An ANOVA table test (done in GWR 3.0,
      R)
    – The Akaike Information Criterion (AIC)
       Less the AIC, better the model
       Rule of thumbs: a decrease of AIC of 3 is
        regarded as successful improvement
                  Tests
   Are the coefficients really varying
    across space
    – F-tests based on the variance of
      coefficients
    – Monte Carlo tests: random permutation of
      the data
          5. An example
   Housing hedonic model in Milwaukee
    – Data: MPROP 2004 – 3430+ samples
      used
    – Dependent variable: the assessed value
      (price)
    – Independent variables: air conditioner,
      floor size, fire place, house age, number
      of bathrooms, soil and Impervious surface
      (remote sensing acquired)
             The global model
                           Estimate        Std. Error    t value   Pr(>|t|)
(Intercept)                18944.05        4112.79       4.61      4.25e-06
Floor Size                 78.88           2.00          39.42     <2e-16
House Age                  -508.56         33.45         -15.20    <2e-16
Fireplace                  14688.13        1609.53       9.13      <2e-16
Air Conditioner            13412.99        1296.51       10.35     <2e-16
Number of Bathrooms 19697.65               1725.64       11.42     <2e-16
Soil&Imp. Surface          -27926.77       5179.42       -5.39     7.44e-08
Residual standard error: 35230 on 3430 degrees of freedom
Multiple R-Squared: 0.6252, Adjusted R-squared: 0.6246
F-statistic: 953.7 on 6 and 3430 DF, p-value: < 2.2e-16
Akaike Information Criterion: 81731.63
        The global model
   62% of the dependent variable’s variation is
    explained
   All determinants are statistically significant
   Floor size is the largest positive
    determinant; house age is the largest
    negative determinant
   Deteriorated environment condition (large
    portion of soil&impervious surface) has
    significant negative impact
        GWR run: summary
 Number of nearest neighbors for
  calibration: 176 (adaptive scheme)
 AIC: 76317.39 (global: 81731.63)
                                       ANOVA Test
    Source                SS                   DF        MS               F
    OLS Residuals         4257667878068.3 7.00
    GWR Improvement       3544862425088.0 327.83         10813043388.63
    GWR Residuals         712805558309.1 3102.17         229776586.89     47.06
    GWR Akaike Information Criterion: 76317.39 (OLS: 8173 1.63)


   GWR performs better than global
    model
           GWR run: non-
         stationarity check
                    F statistic   Numerator DF   Denominator DF*   Pr (> F)
Floor Size          2.51          325.76         1001.69           0.00
House Age           1.40          192.81         1001.69           0.00
Fireplace           1.46          80.62          1001.69           0.01
Air Conditioner     1.23          429.17         1001.69           0.00
Number of Bathrooms 2.49          262.39         1001.69           0.00
Soil&Imp. Surface   1.42          375.71         1001.69           0.00

       Tests are based on variance of coefficients, all
       independent variables vary significantly over space
                                                            Fire Place
Floor Size                           Air Conditioner
                                                              High : 74706.97
 High : 119.49                         High : 55860.63

 Low : 17.63                           Low : -7098.88         Low : -6722.29




     A                                         B                  C




 Num. of Bathrm
                             ±        House Age            Soil & Imp. Sfc
   High : 39931.12                        High : 929.44      High : 34357.96

   Low : -2044.24                         Low : -1402.30     Low : -220301.55




                                             E                        F
             D



                     0   5      10             20
                             Kilometers
     General conclusions
   Except for floor size, the established
    relationship between house values and the
    predictors are not necessarily significant
    everywhere in the City
   Same amount of change in these attributes
    (ceteris paribus) will bring larger amount of
    change in house values for houses locate
    near the Lake than those farther away
    General conclusions
   In the northwest and central eastern
    part of the City, house ages and house
    values hold opposite relationship as
    the global model suggests
    – This is where the original immigrants built
      their house, and historical values weight
      more than house age’s negative impact
      on house values
    6. Interested Groups
   GWR 3.0 software package can be obtained
    from Professor Stewart Fotheringham
    stewart.fotheringham@MAY.IE
   GWR R codes are available from Danlin Yu
    directly (danlinyu@uwm.edu)
   Any interested groups can contact either
    Professor Yehua Dennis Wei
    (weiy@uwm.edu) or me for further info.
       Interested Groups
   The book: Geographically Weighted
    Regression: the analysis of spatially
    varying relationships is HIGHLY
    recommended for anyone who are
    interested in applying GWR in their
    own problems
    Acknowledgement
 Parts of the contents in this workshop
  are from CSISS 2004 summer
  workshop Geographically Weighted
  Regression & Associated Statistics
 Specific thanks go to Professors
  Stewart Fotheringham, Chris
  Brunsdon, Roger Bivand and Martin
  Charlton
 Thank you all

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