[R] a question on statistics (rather than R-specific)

Prof Brian D Ripley ripley at stats.ox.ac.uk
Fri Jun 21 08:37:00 CEST 2002

On Thu, 20 Jun 2002, Boryeu Mao wrote:

> I have used plor() to model a rather large 3-category dataset (~1500 data
> points, ~15 independent variables); from the resulting model (with a
> deviance slightly below the residual degrees of freedom), the training data
> are placed in only the two extreme categories.  Though the result appears to
> indicate that there's only a relative 'narrow' bin for the medium group,
> [and when the dataset is re-organized into 2 categories, glm(family =
> binomial(link = logit) ...) gives a model with a deviance reduced by about
> half from the 3-category polr() result], I am questioning if this (the
> 'narrow-bin') interpretation is too simplistic (or entirely incorrect), and
> wondering about the meaning of the absence of (fitted) data points in the
> medium group.
> Thanks in advance for any hints/pointers!

Probably the POLR model is inappropriate: try multinom for a fair
comparison (you cannot compare likelihoods on grouped and ungrouped data).

See the examples in MASS (the book) which is where polr() comes from.

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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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