Hi there,
this is the output from R
> solb2wvb<-nls(~sqrt(novervar)*(weight-(a*(1-exp(-b*(age-c))))^3),data=solb2.na.rm,start=list(a=0.85,b=0.45,c=0.48))
> summary(solb2wvb)
Formula: ~ sqrt(novervar) * (weight - (a * (1 - exp( - b * (age - c))))^3)
Parameters:
Value Std. Error t value
a 1.087370 0.01193090 91.1392
b 0.151838 0.00714963 21.2372
c -1.809770 0.13186000 -13.7250
Residual standard error: 4.41368 on 109 degrees of freedom
The output from Prism is:
von Bertalanffy
Best-fit values
A 0.8957
B 0.2381
C -1.358
Std. Error
A 0.002280
B 0.002568
C 0.02919
95% Confidence Intervals
A 0.8912 to 0.9001
B 0.2331 to 0.2431
C -1.415 to -1.300
The latter has much better visual fit and reasonable residuals. Furthermore
theory and practice both lead to the expectation that this model should fit the
data.
Incidentally, I was under the impression that with a weighted nls in R the SE
values were not accurate.
Finally I've attached the dataset
-----Original Message-----
From: Liaw, Andy [mailto:[EMAIL PROTECTED]
Sent: 27 January 2005 15:25
To: Robert Brown FM CEFAS; [email protected]
Subject: RE: [R] weighting in nls
Can you show us the difference; i.e., what are the parameter estimates and
associated SEs from the two programs? Even better, can you supply an
example data set?
[With is `trick' for weighted nls, you need to be careful with the output of
predict().]
Andy
> From: Robert Brown FM CEFAS
>
> I'm fitting nonlinear functions to some growth data but I'm
> getting radically different results in R to another program
> (Prism). Furthermore the values from the other program give a
> better fit and seem more realistic. I think there is a
> problem with the results from the r nls function. The
> differences only occur with weighted data so I think I'm
> making a mistake in the weighting. I'm following the
> procedure outlined on p 244 of MASS (or at least I'm trying to).
>
> Thus, I'm using mean data with heteroscedasticity so I'm
> weighting by n/ variance, where the variance is well known
> from a large data set. This weighting factor is available as
> the variable 'novervar'.
>
> The function is a von Bertalanffy curve of the form
> weight~(a*(1-exp(-b*(age-c))))^3. Thus I'm entering the
> command in the form:
>
> solb1wvb<-nls(~sqrt(novervar)*(weight-(a*(1-exp(-b*(age-c))))^
> 3),data=solb1.na.rm,start=list(a=0.85,b=0.45,c=0.48))
>
> Can anyone suggest what I'm doing wrong? I seem to be
> folowing the instructions in MASS. I tried following the
> similar instructions on page 450 of the white book but these
> were a bit cryptic.
>
> I'm using R 2.0.0 on a Windows 2000 machine
>
> Regards,
>
> Robert Brown
>
>
> **************************************************************
> *********************
> This email and any attachments are intended for the named
> re...{{dropped}}
>
> ______________________________________________
> [email protected] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
> http://www.R-project.org/posting-guide.html
>
>
------------------------------------------------------------------------------
Notice: This e-mail message, together with any attachments, contains
information of Merck & Co., Inc. (One Merck Drive, Whitehouse Station, New
Jersey, USA 08889), and/or its affiliates (which may be known outside the
United States as Merck Frosst, Merck Sharp & Dohme or MSD and in Japan, as
Banyu) that may be confidential, proprietary copyrighted and/or legally
privileged. It is intended solely for the use of the individual or entity named
on this message. If you are not the intended recipient, and have received this
message in error, please notify us immediately by reply e-mail and then delete
it from your system.
------------------------------------------------------------------------------
***********************************************************************************
This email and any attachments are intended for the named recipient only. Its
unauthorised use, distribution, disclosure, storage or copying is not
permitted. If you have received it in error, please destroy all copies and
notify the sender. In messages of a non-business nature, the views and
opinions expressed are the author's own and do not necessarily reflect those of
the organisation from which it is sent. All emails may be subject to
monitoring.
***********************************************************************************
______________________________________________
[email protected] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
