## Geographically Weighted Regression

Hui Hu Ph.D.

Department of Epidemiology

College of Public Health and Health Professions & College of Medicine

March 25, 2020

# Geographically Weighted Regression

• If we apply a traditional regression model to spatial data, we usually assume a stationary process
-  the same exposure has the same impact on the outcome in all parts of the study region

• The estimated coefficients are constant over space
-  assume that the values of β are the same everywhere
y_i=\beta_0+\beta_1x_{1i}+...+\beta_nx_{ni}+\varepsilon_i

### Measured Associations Might Vary Spatially

• Sampling variation

• Associations intrinsically different across space
-  e.g. differences in attitudes, different political or other contextual effects

• Model misspecification

### Geographically Weighted Regression

• To address the issue of spatial non-stationary directly and allow the associations to vary over space

• We can then estimate the values of β by:

• W(i) is a weight matrix specific to location i such that observations nearer to i are given greater weight than observations further away
y_i=\beta_{0(i)}+\beta_{1(i)}x_{1i}+...+\beta_{n(i)}x_{ni}+\varepsilon_i
\beta_{(i)}=(X^TW_{(i)}X)^{-1}X^TW_{(i)}Y

### Spatial Weight Functions

• Numerous weight functions can be used
-  usually use Gaussian or "Gaussian-like" functions to reflect the type of dependency found in most spatial processes
-  can be either fixed or adaptive

Fixed

W_{ij}=e^{- {{(d_{ij}/h)^2} \over 2}}

h is the bandwidth, as it increases, the gradient of the kernel becomes less steep and more data points are incuded

W_{ij}= \{
[1-(d_{ij}^2/h^2)]^2
0

if j is one of the Nth nearest neighbors of i

otherwise

### Calibration

• Results of GWR appear to be relatively insensitive to the choice of weighting functions
-  as long as it is a continuous distance-based function

• However, the results will be sensitive to the degree of distance-decay

• Therefore, an optimal value of either h or N has to be obtained
-  through minimizing a cross-validated score or the AIC

### Bandwidth Selection

• Optimal bandwidth selection is a trade-off between bias and variance
-  too small a bandwidth leads to large variance in the local estimates
-  too large a bandwidth leads to large bias in the local estimates

### Output from GWR

• Main output from GWR is a set of location-specific coefficient estimates which can be mapped and analyzed to provide information on spatial non-stationary in associations

• We  can also use GWR to
-  estimate local standard errors
-  derive local t statistics
-  calculate local goodness-of-fit measures
-  perform tests to assess the significance of the spatial variation in the local parameter estimates
-  perform tests to determine if the local model performs better than the global one

By Hui Hu

# PHC6194-Spring2020-Lecture10

Lecture 10, Spring 2020, PHC6194 Spatial Epidemiology

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