[R] confidence bands for a quasipoisson glm

[Maik Rehnus]

Dear all,

I have a quasipoisson glm for which I need confidence bands in a graphic:

gm6 <- glm(num_leaves ~  b_dist_min_new, family = quasipoisson, data = beva)
summary(gm6)

library('VIM')
b_dist_min_new <- as.numeric(prepare(beva$dist_min, scaling="classical", transformation="logarithm")).

My first steps for the solution are following:

range(b_dist_min_new)
x <- seq(-1.496, 1.839, by=0.01)  
newdat <- data.frame(b_dist_min_new=x)
y <- predict(gm6, newdata=newdat, type="response")
plot(x,y, type="l", ylim=c(0,15), lty=2, xlab="Distance [scaled log.]", ylab="Number of used plant", las=1)

ilogit<-function(x) exp(x)/(1 + exp(x))
logit <-function(x) log(x/(1 - x))

newdat$logitpred <- predict(gm6, newdata=newdat, type="link")   
newdat$sepred <- predict(gm6, newdata=newdat, type="link", se.fit=TRUE)$se.fit  
newdat$logitlower <- newdat$logitpred-1.96 * newdat$sepred     
newdat$logitupper <- newdat$logitpred+1.96 * newdat$sepred
newdat$upper <- ilogit(newdat$logitupper)                      
newdat$lower <- ilogit(newdat$logitlower)
lines(x, newdat$lower, lty=3)                                  
lines(x, newdat$upper, lty=3).

In this way I could find a positive correlation. But my created confidence bands on the graph don't touch my regression line. Could it be a technical problem or is it a mistake in the calculation?

I am new here and I hope you can help to solve my problem. I could not find any answers for quasipoisson glm on internet.

Best regards

Maik



  
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