[R] predicting x values in a polynomial regression
Sebastian Leuzinger
Sebastian.Leuzinger at unibas.ch
Wed Oct 27 10:26:02 CEST 2004
Dear R
I would like to predict x-values in a 4th order polynomial regression:
x <- c(1:10)
y <- c(2,7,19,49,89,94,97,98,92,89) # these are percentages
lm(y ~ x+I(x^2)+I(x^3)+I(x^4)-1) -> lm1
now I would like to know what the model fit (x-value) for y=50 is. This
results in solving a 4th order quadratic equation. polyroot() does not
really help because it only gives me the x-values for y=0. I have tried
with nls() which sort of works, but I am sure there is a much easier
solution to that, can anyone give me a hint?
--
Sebastian Leuzinger
Institute of Botany, University of Basel
SchÃ¶nbeinstr. 6 CH-4056 Basel
Ph. 0041 (0) 61 267 3511
fax 0041 (0) 61 2673504
email: Sebastian.Leuzinger at unibas.ch <mailto:Sebastian.Leuzinger at unibas.ch>
web: http://www.unibas.ch/botschoen/leuzinger/e.shtml
<http://www.unibas.ch/botschoen/leuzinger/d.shtml>
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