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

[John McNeil]

Wow, thanks, that does it perfectly

John

On Apr 17, 2007, at 5:51 PM, Gabor Grothendieck wrote:

> Try this:
>
> library(drc)
> # 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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