Hello all. I have heard over and over that CART and its various tree-like
brethren are "non-parametric" techniques. When I read the chapter in
Chambers and Hastie on tree-based models it states that tree-based models
can be generalized (GTMs) in a manner similar to GLMs by specifying a
different deviance function to distributions other than the gaussian error
distribution ( section 9.4.3).  I have an application in which the response
variable is a continuous variable representing tree counts within a unit
area and thus would be best described by a poisson distribution. The error
distribution for this data is not gaussian. If this is the case, will the
gaussian error distribution used in most regression tree packages, be
appropriate? Are there ways to specify the error distribution in R or Should
I log transform the response variable?  If the specification of error
distribution in regression trees is important, than are these techniques
truly " non-parametric". Thanks for your inputs. 

Solomon Dobrowski
Tahoe Environmental Research Center (TERC)
John Muir Institute of the Environment
University of California, Davis
530 754 9354

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