Hi
your model
f <- function(x,a,b) {a * I(x^b)}
can be expressed as
log(a)+b*log(x)
and for that it shall result in straight line and you can use lm for estimate
of b and log(a)
It is also better to use 1:33 instead of 1980:2012
Based on values you get from linear realation you can set sensible starting
values.
> fm <- nls(y~a*z^b, data=test, start=c(a=0.2,b=2))
> summary(fm)
Formula: y ~ a * z^b
Parameters:
Estimate Std. Error t value Pr(>|t|)
a 0.13391 0.01848 7.248 3.74e-08 ***
b 2.22563 0.04140 53.763 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 6.239 on 31 degrees of freedom
Number of iterations to convergence: 6
Achieved convergence tolerance: 7.673e-07
Regards
Petr
> -----Original Message-----
> From: r-help-boun...@r-project.org [mailto:r-help-bounces@r-
> project.org] On Behalf Of muzammil786
> Sent: Wednesday, November 14, 2012 12:12 PM
> To: r-help@r-project.org
> Subject: Re: [R] R exponential regression
>
> I am having a similar problem on this data (given below). I have tried
> several starting values for a and b, but it is consistently giving me:
>
> fm <- nls(y~f(x,a,b), data.frame(x,y), start=c(a=1,b=1)) Error in
> numericDeriv(form[[3L]], names(ind), env) :
> Missing value or an infinity produced when evaluating the model
>
> The function f(x,a,b) is defined here:
> f <- function(x,a,b) {a * I(x^b)}
>
> The data is given here:
>
> x y
> 1 1980 1
> 2 1981 1
> 3 1982 1
> 4 1983 3
> 5 1984 3
> 6 1985 5
> 7 1986 8
> 8 1987 8
> 9 1988 9
> 10 1989 12
> 11 1990 15
> 12 1991 24
> 13 1992 33
> 14 1993 44
> 15 1994 62
> 16 1995 68
> 17 1996 81
> 18 1997 87
> 19 1998 102
> 20 1999 114
> 21 2000 123
> 22 2001 135
> 23 2002 144
> 24 2003 158
> 25 2004 172
> 26 2005 188
> 27 2006 197
> 28 2007 224
> 29 2008 234
> 30 2009 254
> 31 2010 278
> 32 2011 312
> 33 2012 317
>
> I shall be grateful if you could guide me what's wrong here.
>
> Cheers.
> Muzammil
>
>
>
> --
> View this message in context: http://r.789695.n4.nabble.com/R-
> exponential-regression-tp1009449p4649474.html
> Sent from the R help mailing list archive at Nabble.com.
>
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