[R] convergence in polr

C. Spanou cs369 at cam.ac.uk
Tue Feb 24 16:30:17 CET 2004

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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