Re: [R] How to understand output from R's polr function (ordered logistic regression)?

From: Dan Frankowski <dfrankow_at_gmail.com>
Date: Tue, 01 Mar 2011 15:29:46 -0600

Also posted as
http://stats.stackexchange.com/questions/7720/how-to-understand-output-from-rs-polr-function-ordered-logistic-regression .

Also, I read section 7.3 of "Modern Applied Statistics with S" by Venables and Ripley (who wrote polr?), and I can still not answer many of these questions.

On Tue, Mar 1, 2011 at 3:25 PM, Dan Frankowski <dfrankow_at_gmail.com> wrote:

> I am new to R, ordered logistic regression, and polr.
>
> The "Examples" section at the bottom of the help page for polr<http://stat.ethz.ch/R-manual/R-patched/library/MASS/html/polr.html>(that fits a logistic or probit regression model to an ordered factor
> response) shows
>
> options(contrasts = c("contr.treatment", "contr.poly"))
>
> house.plr <- polr(Sat ~ Infl + Type + Cont, weights = Freq, data = housing)
>
> pr <- profile(house.plr)
>
> plot(pr)
> pairs(pr)
>
>
> -
>
> What information does pr contain? The help page on profile<http://stat.ethz.ch/R-manual/R-patched/library/stats/html/profile.html>is generic, and gives no guidance for polr.
> -
>
> What is plot(pr) showing? I see six graphs. Each has an X axis that is
> numeric, although the label is an indicator variable (looks like an input
> variable that is an indicator for an ordinal value). Then the Y axis is
> "tau" which is completely unexplained.
> -
>
> What is pairs(pr) showing? It looks like a plot for each pair of input
> variables, but again I see no explanation of the X or Y axes.
> -
>
> How can one understand if the model gave a good fit? summary(house.plr)shows Residual Deviance 3479.149 and AIC (Akaike Information Criterion?) of
> 3495.149. Is that good? In the case those are only useful as relative
> measures (i.e. to compare to another model fit), what is a good absolute
> measure? Is the residual deviance approximately chi-squared distributed? Can
> one use "% correctly predicted" on the original data or some
> cross-validation? What is the easiest way to do that?
> -
>
> How does one apply and interpret anova on this model? The docs say
> "There are methods for the standard model-fitting functions, including
> predict, summary, vcov, anova." However, running anova(house.plr)results in anova
> is not implemented for a single "polr" object
> -
>
> How does one interpret the t values for each coefficient? Unlike some
> model fits, there are no P values here.
>
> I realize this is a lot of questions, but it makes sense to me to ask as
> one bundle ("how do I use this thing?") rather than 7 different questions.
> Any information appreciated.
>

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