From roger at ysidro.econ.uiuc.edu  Sun Jul 30 17:29:37 2006
From: roger at ysidro.econ.uiuc.edu (roger koenker)
Date: Sun, 30 Jul 2006 10:29:37 -0500
Subject: [Rd] Parametric links for glm?
Message-ID: <0EA0A52D-EA2C-459D-8D39-205A9FDE5F5C@ysidro.econ.uiuc.edu>

At useR 2006 I mentioned that it would be nice to have a way to  
specify binomial links
that involved free parameters  and described some experience with a  
Gosset link involving
a free degrees of freedom parameter, and a Tukey-lambda link with two  
free parameters.
My implementation of this involved some rather kludgey modifications  
of binomial,
make.link and glm that (essentially) added a "..." argument to  
binomial permitting the
parameters to be passed down to the fitting routines.  So, in the  
Gosset case, for example,
had the following passage in  make.link, involving a free degrees of  
freedom parameter nu:

}, Gosset = {
         linkfun <- function(mu) qt(mu,nu)
         linkinv <- function(eta) {
             thresh <- -qqt(.Machine$double.eps,nu)
             eta <- pmin(thresh, pmax(eta, -thresh))
             pt(eta,nu)
         }
         mu.eta <- function(eta) pmax(dt(eta,nu), .Machine$double.eps)
         valideta <- function(eta) TRUE
},

Just prior to the useR meeting, there were some changes in R-devel to  
binomial that allow one to pass an object of class "link-glm" -- if I  
understand properly, this allows one to more easily
roll one's own links, but it leaves the question of passing  
parameters on the table.  That is,
presumably, I could pass a structure, like that above of class "link- 
glm" and thereby avoid
the Cerberus of  officially sanctioned links, but I still don't see  
how to pass my parameters.
Can someone enlighten me?  Or,  would it still be necessary to add  
"..." to the binomial
function?

url:    www.econ.uiuc.edu/~roger                Roger Koenker
email   rkoenker at uiuc.edu                       Department of Economics
vox:    217-333-4558                            University of Illinois
fax:    217-244-6678                            Champaign, IL 61820



[Rd] Parametric links for glm?
roger koenker