[R] a question on statistics (rather than R-specific)
B.Mao at cerep.com
Fri Jun 21 17:02:21 CEST 2002
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.)
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
> are placed in only the two extreme categories. Though the result appears
> 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),
> 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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