R-helpers,

I am using the package "drc" to fit a 4 parameter logistic model.  When I 
use the predict function to get prediction on a new dataset, I am not 
getting the requested confidence or prediction intervals.  Any idea what 
is going on?  Here is code to reproduce the problem:

---

library(drc)

# Fit model to existing dataset in package
spinach.model <- drm(SLOPE~DOSE, data = spinach, fct = LL.4())

#Generate new fake dataset
newdt <- data.frame(matrix(c(seq(0, 150, 0.1), rep(NA, 1501)), ncol=2, 
byrow=F))
colnames(newdt) <- c('DOSE', 'SLOPE')

#Use predict function to get prediction and confidence intervals
pred <- predict(spinach.model, interval='prediction', newdata=newdt)
conf <- predict(spinach.model, interval='confidence', newdata=newdt)
head(pred); head(conf)

---

Examining the output shows the point estimates but not the intervals. 
Would like the intervals.


I am using the most recent versions of R and drc on Windows XP.

Thanks,

Brant
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