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

Prof Brian D Ripley ripley at stats.ox.ac.uk
Fri Jun 21 17:49:46 CEST 2002

```On Fri, 21 Jun 2002, Boryeu Mao wrote:

> Thanks for the reply.  Since the 3 categories are ordinal (the data are
> grouped into the categories from continuous variable), what would be the
> appropriate function for 2-category ordered data?  (though I will try
> multinom() on the data.)

Is it two or three categories?  A multinomial model is always appropriate,
but a POLR model makes a strong proportionality assumption which may or
may not be appropriate.

As I said, there is a worked example in MASS (the book), including
examining the POLR assumption.

>
> -----Original Message-----
> From: Prof Brian D Ripley [mailto:ripley at stats.ox.ac.uk]
> Sent: Thursday, June 20, 2002 11:37 PM
> To: Boryeu Mao
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] a question on statistics (rather than R-specific)
>
>
> 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
>

--
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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```