[R] nls

[Bert Gunter]

On Fri, Feb 25, 2011 at 6:09 AM, Abeer Fadda
<a.fadda at zmbh.uni-heidelberg.de> wrote:
> hi,
> I would like to find the x value (independent variable) for a certain dependent value using the fitted model with nls.
> with (predict) I can find y that corresponds to a list of x. I need the other way around. can it be done?
> thanks,
> afadda

Yes.

-- Bert




... Oh, if you mean HOW can it be done? -- lots of ways. Analytically,
if y=f(x) is your fitted model, just backsolve x = g(y).

If your algebra isn't up to the task, then just predict y on a
suitably fine x grid, and (assuming monotonicity) find the x
corresponding to the predicted y closest to the y you wish to back
calibrate. This is just a matter of indexing. See ?which.




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-- 
Bert Gunter
Genentech Nonclinical Biostatistics