[R] polr problem solved

[Prof Brian Ripley]

On Fri, 8 Oct 2004, Peter Flom wrote:

> I'd like to thank John Fox and Chuck Cleland for their help in resovling
> this issue.  It turned out to be something simple, but perhaps others
> have had similar problems
> 
> In my original data frame, I had 4 categories of race/ethnicity.  One of
> the categories (other) was very small, and not similar to any of the
> other three categories, so I created a new data frame deleting those
> people.
> 
> However, the level "other" was still there, with no one in it.
> This didn't cause a problem for glm or lm, but it did for polr.  When I
> eliminated that level, the problem disappeared.

How did you use `glm or lm' for an order factor response?  An empty factor 
level will certainly cause glm problems, depending which one it is.

An empty level will always cause polr problems, as there is no MLE under 
those circumstances.  I will add a sanity check in due course.

-- 
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