Here is more information on the equation. It is a growth function:

Growth = a + b*(1-exp(-k*time))

where a, b and k are parameters. I wanted to test the difference in total growth between treatments and the parameters a + b represent total growth. Thus, I figured that I could add the parameters and the SE to test the treatment differences. I also need the rate constant k and the initial values a and their treatment differences.
I found the equation I posted on this website:
http://sci.tech-archive.net/Archive/sci.math/2004-07/4780.html
This book was referenced: Kendall & Stuart's Advanced Theory of Statistics

Thanks for all you your help.
Adele


From: ml-node+3477116-1659458804-211414@n4.nabble.com [mailto:ml-node+3477116-1659458804-211414@n4.nabble.com]
Sent: Tuesday, April 26, 2011 09:29 PM
To: Thompson, Adele - Adele_Thompson@cargill.com
Subject: Re: How can I extract information from list which class is nls

On Tue, Apr 26, 2011 at 2:21 PM, Schatzi <[hidden email]</user/SendEmail.jtp?type=node&node=3477116&i=0&by-user=t>> wrote:

> How do I extract the standard error of the parameter estimates?
>
> Also, if I would like to add two parameters together (x+y), can I use this
> equation to calculate the new standard error?:
> x = parameter 1
> y = parameter 2
> xSE = SE parameter 1
> ySE = SE parameter 2
>
> NewSE=(x+y)*sqrt((xSE/x)^2+(ySE/y)^2)

Try taking the square roots of their variance:

> example(nls)
> sqrt(diag(vcov(fm1DNase1)))
      Asym       xmid       scal
0.07815395 0.08135321 0.03227080
>
> # it gives the same result as another solution
> # offered on this thread
> coef(summary(fm1DNase1))[, "Std. Error"]
      Asym       xmid       scal
0.07815395 0.08135321 0.03227080


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