[R] polr problem solved

[Peter Flom]

Prof Brian Ripley <ripley at stats.ox.ac.uk> 10/09/04 3:18 AM asked

<<<
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.
>>>


The analyses were part of a paper I am writing, illustrating that, when
the DV is oddly
distributed (the DV in question was a count, with many 0's, and a long
right tail) that
the 'usual' methods not only are wrong for statisically reasaons (such
as grossly violating
model assumptions) but also give bad results.

While this is widely known to statisticians, in the fields in which I
work, people sometimes
analyze such variables using either OLS regression (hence lm), or by
categorizing the DV into 
something like 0, 1, 2, more than 2 (hence the need for polr). I also
tried Poisson regression and
negative binomial regression (hence glm).  

The empty level of the IV only caused a problem for polr

Thanks 

Peter


Peter

Peter L. Flom, PhD
Assistant Director, Statistics and Data Analysis Core
Center for Drug Use and HIV Research
National Development and Research Institutes
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