R help - Sep 2007 - [Help] Error when using nls

Hi,

     I am a student of Earthquake Engineering, and am new to R.  
Currently I try to run nonlinear regression analysis by R. My data  
has three variables: X, Y, and Z. Z is a function of (X, Y). My R  
script is as below.


rm(list=ls())
# read in data
Alldata <- read.table("~/Documents/R/Wu_data.dat", header=TRUE)
# assign variables
Z <- Wu[[1]]     # N1,60
X <- Wu[[2]]   # CSR
Y <- Wu[[3]]    # ev, %

# set the data frame
data1<- data.frame(cbind(N,CSR,ev))

# call nls funciton
out <- nls(Z~X/(1/(a+b*ev+c*(Y^2))+X/(d+e*exp(f*Y)))-g, data1,  
selfStart, trace=T)


In total there are 59 sets of (X,Y,Z). When I run this script, I got  
the error message as:

parameters without starting value in 'data': a, b, d, e, f, g


I don't know what this means. Besides, I use "selfStart" option,
such
that the nls is supposed to run by finding the initial values of  
(a,b,c,d,e,f,g) itself. Why the nls asks me to give the starting values?

What should I do next? Does anyone can explain the meaning of the  
error message?
Any comment is appreciated. Thanks a lot.


Regards,


Niner
----------------------------------------------------
Niner, Seattle
ninerdummy@gmail.com




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On Sep 29, 2007, at 12:00 AM, Niner wrote:
> Hi,
>
>      I am a student of Earthquake Engineering, and am new to R.
> Currently I try to run nonlinear regression analysis by R. My data
> has three variables: X, Y, and Z. Z is a function of (X, Y). My R
> script is as below.
In general it is recommended that you provide reproducible code,  
typically using one of the built in data sets instead of a data set  
of your own that we have no access to.
> # call nls funciton
> out <- nls(Z~X/(1/(a+b*ev+c*(Y^2))+X/(d+e*exp(f*Y)))-g, data1,
> selfStart, trace=T)
>
>
> In total there are 59 sets of (X,Y,Z). When I run this script, I got
> the error message as:
>
> parameters without starting value in 'data': a, b, d, e, f, g
>
>
> I don't know what this means. Besides, I use "selfStart"
option, such
> that the nls is supposed to run by finding the initial values of
> (a,b,c,d,e,f,g) itself. Why the nls asks me to give the starting  
> values?
"selfStart" is not an "option", it is a function which you
would use
to construct the self-starting object. Look at ?selfStart for an  
example on how to use it.
To the best of my understanding, you need to either:
1) provide starting values (what I would recommend, if you have a  
sense for the range), or
2) provide R with a mechanism for finding the starting values (via  
constructing a selfStart object, see ?selfStart), or
3) ask R to make very poor guesses, by leaving the start argument  
empty [  out <- nls(Z~X/(1/(a+b*ev+c*(Y^2))+X/(d+e*exp(f*Y)))-g,  
data1, trace=T)   ]
which has a high chance of choking out on you. In an example I just  
tried, R set all the parameter values to 1.
> What should I do next? Does anyone can explain the meaning of the
> error message?
> Any comment is appreciated. Thanks a lot.
>
>
> Regards,
> Niner
> ----------------------------------------------------
> Niner, Seattle
> ninerdummy at gmail.com
>
Haris Skiadas
Department of Mathematics and Computer Science
Hanover College