Hi All,
 
I am constructing "gravity" (spatial interaction) models in R for
ecological application.  I have linearized the unconstrained model and
solved using OLS.  However, I would like to do some further, more
in-depth, analyses.
 
1)     Is there an R package actually designed for gravity models?  I
have done several searches on the CRAN site with no luck.  My preference
would be to use R for this manuscript versus commercially available
software.  
2)     I would like to estimate my linearized unconstrained model using
maximum likelihood.  This may be a simple solution, but I have yet to
figure out how to do this (my apologies if this is something I should
have been able to figure out on my own).
3)     For a production (origin) constrained model, I can linearize the
equation as in the unconstrained model.  The models are similar, accept
I need to estimate a constant (k) for each origin, but with only one
estimate of the remaining parameters.  Any ideas on how to
operationalize this?  I have tried exploring options with multiple
regression, but am not finding a solution with the appropriate
mechanics.  This would also be an ML estimate.  
4)     Any feedback/experience from people who have used gravity models
in R and/or ecological application would be appreciated.
5)     I have tried to keep this posting as brief as possible.  Please
let me know if you need further information to answer my questions.
 
Thank you all in advance
 
 
Melanie Murphy
EPA-STAR Fellow
School of Biological Sciences
Washington State University
e-mail: mamurphy@wsu.edu
 

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