[R] convergence in polr

[Prof Brian Ripley]

Why have you sent a message about S-PLUS to R-help, one that has already 
been answered on S-news?

There is no function nlminb in R.

On 24 Feb 2004, C. Spanou wrote:

> Hello splus-users, I am trying to fit a regression model for an ordered 
> response factor. So I am using the function polr in library(MASS). My data 
> is a matrix of 1665 rows and 63 columns (one of the column is the dependent 
> variable). The code I use is polr(as.ordered(q23p)~.,data=newdatap)
>  but I am getting the following warning message singularity encountered in: 
> nlminb.1(temp, p, liv, lv, objective, gradient, bounds, scale)
> 
> I looked in the MASS help for nlminb and I found that for the function
> nlminb(start, objective, gradient=NULL, hessian=NULL,  
>        scale=1, control=NULL, lower=-Inf, upper=Inf) 
>  
> 
> when returning a warning message of singularity means that the optimization 
> algorithm thinks it can't make any further progress because it has too many 
> degrees of freedom. It usually means that the objective function is either 
> not differentiable, or it may not have an optimum.
> 
> So for my data an optimum can't be obtained.
> Is this true?
> 
> Can I ignore this warning message since what I want to find is values for 
> the boundaries? Will the values for the boundaries be accurate even though 
> I get the warning message?
> 
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> 

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595