Re: [R] Weibull distribution

From: Thomas Lumley <tlumley_at_u.washington.edu>
Date: Sat 22 Jul 2006 - 01:35:01 EST

On Fri, 21 Jul 2006, Valentin Dimitrov wrote:

> Dear Leaf,
>
> I modified your code as follows:
>
> gamma.fun <- function(mu,sd,start=100)
> {
> f.fn <- function(alpha)
> {abs(sd^2-mu^2/(gamma(1+1/alpha))^2*(gamma(1+2/alpha)-(gamma(1+1/alpha))^2))}
> alpha <- optim(start, f.fn)
> beta <- mu/gamma(1+1/alpha$par)
> return(list=c(a=alpha$par,b=beta));
> }
>
> Now it works properly.
>
> First, I added an abs(). You tried to solve an
> equation by means of the R-function optim(), which
> finds a minimum. That's why you can find the solution
> of f(x)=a through minimization of abs(f(x)-a).
> Second, I deleted the optim-method BFGS from the
> optim() function, because it is not appropriate in
> this case.

optim() is not appropriate at all in this case -- its help page says to use optimize() for one-dimensional problems.

In fact, in one dimension there isn't any need to resort to optimization when you really want root-finding, and uniroot() is more appropriate than optimize().

         -thomas



R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Sat Jul 22 01:58:04 2006

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.1.8, at Fri 04 Aug 2006 - 08:17:34 EST.

Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help. Please read the posting guide before posting to the list.