[R] help: exponential fit?

Lüdde Mirko
2001-05-23T15:17:38Z

Thanks!

I get the impression that, since in SSbiexp
the exponential constants are provided via their logarithms,
both constants are assumed negative,
i.e. the exponentials are decaying?

Is it possible to have a linear combination of a
decaying and a growing exponential?

Cheers, Mirko.


> -----Urspr?ngliche Nachricht-----
> Von: Douglas Bates [mailto:bates at stat.wisc.edu]
> Gesendet: Mittwoch, 23. Mai 2001 14:51
> An: L?dde Mirko
> Cc: 'r-help at stat.math.ethz.ch'; Ralph Schwarzwald (E-Mail)
> Betreff: Re: [R] help: exponential fit?
>
>
> L?dde Mirko <mirko.luedde at cellcontrol.de> writes:
>
> > I'm quite new to R (and statistics),
> > and I like it (both)!
> > But I'm a bit lost in all these packages,
> > so could someone please give me a hint
> > whether there exists a package for fitting
> > exponential curves (of the type
> > t --> \sum_i a_i \exp( - b_i t))
> > on a noisy signal?
> > In fact monoexponential decay + polynomial growth
> > is what I'd like to try.
>
> See the example for the biexponential model, SSbiexp, in the nls
> package for a start.
>
> > example(SSbiexp, package = "nls")
>
> SSbixp> data(Indometh)
>
> SSbixp> Indo.1 <- Indometh[Indometh$Subject == 1, ]
>
> SSbixp> fm1 <- nls(conc ~ SSbiexp(time, A1, lrc1, A2, lrc2),
>     data = Indo.1)
>
> SSbixp> summary(fm1)
>
> Formula: conc ~ SSbiexp(time, A1, lrc1, A2, lrc2)
>
> Parameters:
>      Estimate Std. Error t value Pr(>|t|)
> A1     2.0293     0.1099  18.464 3.39e-07 ***
> lrc1   0.5794     0.1247   4.648  0.00235 **
> A2     0.1915     0.1106   1.731  0.12698
> lrc2  -1.7878     0.7871  -2.271  0.05737 .
> ---
> Signif. codes:  0  `***'  0.001  `**'  0.01  `*'  0.05  `.'
> 0.1  ` '  1
>
> Residual standard error: 0.04103 on 7 degrees of freedom
>
> Correlation of Parameter Estimates:
>             A1   lrc1    A2
> lrc1  0.002546
> A2   -0.424384 0.8771
> lrc2 -0.455538 0.7708 0.939
>
>