Puzzled. Why are the data you offer to predict() for the independent
variable, conc, all NA's? Is there something reversed or inverted
about how drc functions handle formulas.
--
David.
On Nov 28, 2010, at 2:33 PM, Brant Inman wrote:
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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David Winsemius, MD
West Hartford, CT
______________________________________________
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and provide commented, minimal, self-contained, reproducible code.
