R-helpers,

I recently submitted a help request for the predict.drm function found in the drc package.  I am still having issues with the function and I am submitting reproducible code hoping that somebody can help me figure out what is going on.

--------
library(drc)

# Fit a 4 parameter logistic model to ryegrass dataset
fit <- drm(rootl ~ conc, data = ryegrass, fct = LL.4())
summary(fit)

# Generate a fake dataset for prediction
newdt <- data.frame( matrix(c(seq(0.2, 9, 0.01), rep(NA,881)), ncol=2))
	colnames(newdt) <- c('rootl', 'conc')

# Generate prediction intervals and confidence intervals
prd.p <- predict(fit, newdata=newdt, interval='prediction')
prd.c <- predict(fit, newdata=newdt, interval='confidence')

# Check output
> head(prd.p)
     Prediction Lower Upper
[1,]   7.790812    NA    NA
[2,]   7.790476    NA    NA
[3,]   7.790106    NA    NA
[4,]   7.789702    NA    NA
[5,]   7.789262    NA    NA
[6,]   7.788784    NA    NA  

> head(prd.c)
     Prediction Lower Upper
[1,]   7.790812    NA    NA
[2,]   7.790476    NA    NA
[3,]   7.790106    NA    NA
[4,]   7.789702    NA    NA
[5,]   7.789262    NA    NA
[6,]   7.788784    NA    NA

--------

There appears to be a problem with the predict.drc function.  This code previous generated confidence and prediction intervals in columns 2 and 3 of the prediction matrices but now fails for reasons unknown to me.  Anyone have an idea of what is going on and how I can remedy the situation?

Brant Inman



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