On Fri, Feb 25, 2011 at 6:09 AM, Abeer Fadda <a.fa...@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. > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- Bert Gunter Genentech Nonclinical Biostatistics ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.