[R] help: exponential fit?

[Lüdde Mirko]

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. 


> -----Ursprüngliche Nachricht-----
> Von: Douglas Bates [mailto:bates at stat.wisc.edu]
> Gesendet: Mittwoch, 23. Mai 2001 14:51
> An: Lüdde Mirko
> Cc: 'r-help at stat.math.ethz.ch'; Ralph Schwarzwald (E-Mail)
> Betreff: Re: [R] help: exponential fit?
> 
> 
> Lüdde 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
> 
> 
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