[R] newbie polr() question

[Max]

Prof Brian Ripley used his keyboard to write :
> On Mon, 26 Nov 2007, Max wrote:
>
>> Prof Brian Ripley explained :
>>> On Mon, 26 Nov 2007, Max wrote:
>>> 
>>>> Hi everyone, I'm trying to understand some R output here for ordinal
>>>> regression. I have some integer data called "A" split up into 3 ordinal
>>>> categories, top, middle and bottom, T, M and B respectively.
>>>> 
>>>> I have to explain this output to people who have a very poor idea about
>>>> statistics and just need to make sure I know what I'm talking about
>>>> first.
>>>> 
>>>> Here's the output:
>>>> 
>>>> Call:
>>>> polr(formula = Factor ~ A, data = a, Hess = TRUE, method = "logistic")
>>>> 
>>>> Coefficients:
>>>> Value        Std. Error  t value
>>>> A -0.1259028 0.04758539  -2.645829
>>>> 
>>>> Intercepts:
>>>> Value Std. Error t value
>>>> B|M -2.5872 0.5596 -4.6232
>>>> M|T 0.3044 0.4864 0.6258
>>>> 
>>>> Residual Deviance: 204.8798
>>>> AIC: 210.8798
>>>> 
>>>> I really am not sure what the intercepts mean at all. However, my
>>>> understanding of the coefficient of A is that as the category
>>>> increases, A decreases? If I have an A value of 10, how to I figure out
>>>> the estimated probability that this score is in one of the three
>>>> categories?
>>> 
>>> Use predict(): see the book polr supports for examples (and the theory).
>> 
>> I appreciate the reply, but have difficulty understanding what you mean
>> by "the book polr supports"? :-?
>> 
>> The manuals in R don't reference the polr() command, nor do they write
>> about ordinal regression in R. (from what I can tell) The documentation
>> of the polr() doesn't explain the output or the theory... I've done web
>
> It _does_ describe both the theory and the use of predict(), on pages 
> 204-5.

Is this in your textbook? I found the explanation for predict on page 
1194 of the R reference manual pdf from the help menu in the GUI. 
Unfortunately I do not have the MASS textbook itself, and have to rely 
on whatever I can find about polr() from web.
>
>> searches on polr() and the MASS library and have found little of direct
>> help to my question.
>
> polr is in the MASS package (although you failed to mention that, or give 
> credit).  From library(help=MASS)
>
> BundleDescription:   Functions and datasets to support Venables and
>                       Ripley, 'Modern Applied Statistics with S' (4th
>                       edition).
> and from ?polr:
>
> References:
>
>       Agresti, A. (2002) _Categorical Data._ Second edition.  Wiley.
>
>       Venables, W. N. and Ripley, B. D. (2002) _Modern Applied
>       Statistics with S._ Fourth edition.  Springer.

Thanks guys for the help. I'm still very new to R and didn't even know 
the library(help=MASS) command, I was googling things before... And I 
apologize for not properly crediting you Prof. Ripley, I'm quite new to 
this newsgroup as well, no insult was intended, I just didn't know any 
better.

I'll see about finding your book, I'm hopeful I can convince my 
employer to purchase me some R books. That or I'll spend my own money, 
as it seems a couple texts on R would be extremely useful.