R help - Nov 2008 - Inference and confidence interval for a restricted cubic spline function in a hurdle model

Dear list,

I'm currently analyzing some count data using a hurdle model. I've used 
the rcspline.eval function in the Hmisc-library to contruct the spline 
terms for the regression model, and what I want in the end is the ability 
to compute coefficients and confidence intervals for different changes in 
the smooth function as well as plotting the smooth function along with the 
confidence interval at the values of the x-variable.

An example using the "zero"-part of the hurdle model:

library(Hmisc)
library(pscl)

# Simulate some data

set.seed(1)
y<-c(rep(0,50),rnbinom(50,0.9,0.2))
x<-sin(y)+rnorm(100)

# Set up the spline terms
ssp<-rcspline.eval(x,inclx=T)

# Fit the model and construct the smooth function
f<-hurdle(y~ssp)
knots<-attr(ssp,"knots")
coef<-f$coefficients$zero
w<-rcspline.restate(knots,coef)
fun<-eval(attr(w,"function"))

The coefficient for a change in x from -0.1 to 0.1 is fun(0.1)-fun(-0.1). 
My question is therefore how do I compute the confidence interval for this 
change?

This is easy to do with the Design-library for the "zero"-part but as
far
as I know, zero-truncated negative binomial data can't be fitted using 
Design's functions as they are. Does someone know any neat tricks that 
would make this possible?

Any help on this would be greatly appreciated. Thanks for your time.

/Erik Lampa, Statistician, Uppsala University Hospital, Sweden

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