Hi CR, Assuming your data are stored in "d", perhaps something like this:
m <- nls(log(proc) ~ k + a*(b^cls), start = list(a = 2, b = .25, k =
0), data = d)
summary(m)
# gives
## Formula: log(proc) ~ k + a * (b^cls)
## Parameters:
## Estimate Std. Error t value Pr(>|t|)
## a 5.24617 0.50905 10.306 1.90e-09 ***
## b 0.69640 0.07718 9.023 1.73e-08 ***
## k 1.93055 0.42331 4.561 0.00019 ***
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## Residual standard error: 0.6967 on 20 degrees of freedom
## Number of iterations to convergence: 8
## Achieved convergence tolerance: 4.268e-06
and you can examine the fit:
plot(fitted(m), log(d$proc))
which looks not too bad (see attached)
Cheers,
Joshua
On Sun, Apr 21, 2013 at 12:34 PM, catalin roibu <catalinro...@gmail.com> wrote:
> Hello all!
> I have a problem with a double exponential equation.
> This are my data's> structure(list(proc = c(1870.52067384719,
> 766.789388745793, 358.701545859122, 237.113777545511, 43.2726259059654,
> 148.985133316262, 92.6242882655781, 88.4521557193262, 56.6404686159112,
> 27.0374477259404, 34.3347291080268, 18.3226992991316, 15.2196612445747,
> 5.31600719692165, 16.7015717397302, 16.3923389973684, 24.2702542054496,
> 21.247247993673, 18.3070717608672, 2.8811892177331, 3.18018869564679,
> 8.74204132937479, 7.11596966047229 ), cls = c(0.25, 0.5, 0.75, 1, 1, 1.5,
> 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10)),
> .Names = c("proc", "cls"), row.names = c("0.25", "0.5", "0.75", "1", "11",
> "1.5", "2", "2.5", "3", "3.5", "4", "4.5", "5", "5.5", "6", "6.5", "7",
> "7.5", "8", "8.5", "9", "9.5", "10"), class = "data.frame")
> I want to compute a double exponential equation like this:
> proc=a*exp(b*class)+c*exp(d*class)
>
> Thank you very much for your help!
>
> Best regards!
>
> CR
> --
> ---
> Catalin-Constantin ROIBU
> Lecturer PhD, Forestry engineer
> Forestry Faculty of Suceava
> Str. Universitatii no. 13, Suceava, 720229, Romania
> office phone +4 0230 52 29 78, ext. 531
> mobile phone +4 0745 53 18 01
> +4 0766 71 76 58
> FAX: +4 0230 52 16 64
> silvic.usv.ro
>
> [[alternative HTML version deleted]]
>
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
--
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://joshuawiley.com/
Senior Analyst - Elkhart Group Ltd.
http://elkhartgroup.com
<<attachment: fit.png>>
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