[R] how to estimate dose from respond given drc package result

Gabor Grothendieck ggrothendieck at gmail.com
Wed Apr 18 02:51:07 CEST 2007

Try this:

# get dose corresponding to response (y = 2)
model1 <- multdrc(rootl ~ conc, data = ryegrass)
f <- function(x, y) y - predict(model1, data.frame(conc = x))[1]
uniroot(f, c(0, 10), y = 2)

On 4/17/07, John McNeil <jam at xardas.com> wrote:
> Dear all,
> I can use the very nice drc package (multdrc()) to model and plot a
> dataframe containing dose and response values. I can also use
> predict.drc() to yield response values given a dose. I need to do the
> opposite, estimate a dose given the response. The general predict
> documentation seems to say that this is possible, but it does not
> appear that predict.drc has that capability. This makes sense because
> it is easy to compute y values from x values, given the fitted
> equation, but it is hard to do the opposite.
> It seems that I need to do some kind of successive approximation. Is
> there a package that takes a multdrc generated model and estimates
> dose given response?
> Thank you all for your time,
> John
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