[R] Fwd: Help with nonlinear regression

[Ahmed Attia]

---------- Forwarded message ----------
From: Ahmed Attia <ahmedatia80 at gmail.com>
Date: Mon, Jan 14, 2013 at 2:31 AM
Subject: Help with nonlinear regression
To: r-help at r-project.org





Dears-

I have a question about nonlinear regression.

Asymptotic Regression Model


 below is the equation of the model  (Reference the R book page 662)


y = a - b e^-cx when I compensate x with 50 for instance, I get a very
high value does not match the graph produced by the same model
(Asymptotic regression model).

below is the output analysis from R;

Formula: GN1 ~ SSasymp(nrate, a, b, c)

Parameters:
  Estimate Std. Error t value Pr(>|t|)
a  12.8190     0.4407   29.09 8.92e-05 ***
b   5.7249     0.2803   20.42 0.000257 ***
c  -4.6697     0.1703  -27.41 0.000107 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 0.2874 on 3 degrees of freedom

Number of iterations to convergence: 0
Achieved convergence tolerance: 7.184e-06

>

based on these coefficients  should y= 12.81 – 5.72 e4.66x   ?

the predicted lines indicate that the model works fine, but I do not
know how to compensate x in the equation to get y



AA





--
Ahmed M. Attia


Research Assistant
Dept. Of Soil&Crop Sciences
Texas A&M University
ahmed.attia at ag.tamu.edu
Cell phone: 001-979-248-5215