R help - Aug 2005 - Manually Calculating Odds from POLR Model

Hello,

I am using polr(...) to generate a model.  The summary shows the 
coefficients and the intercepts.

For example:

    coefficient for x1 = c1
    coefficient for x2 = c2

    intercept A|B = i1
    intercept B|C = i2

I can then run predict(..., type="p") with the model and see the odds
for
each factor.

For example:

      A        B        C
1    0.3     0.5      0.2
2    0.4     0.1      0.5

What I really want to be able to do is take the 2 coefficients, the 2 
intercepts, the x1 & x2 values and manually calculate the probabilities 
generated by predict().

I have been searching quite extensively for the underlying calculations that 
transform the polr output and the input variables into the final output 
odds.  I have tried a number of dead-end roads so far.

So, if anyone has any information on how to do this or where I can find out, 
I would be extremely grateful.

Thank you for your time,
Tate Avery

Dear Tate,

If I understand correctly what you're asking, the formulas are on p. 21 of
the paper at
<http://socserv.socsci.mcmaster.ca/jfox/Papers/logit-effect-displays.pdf>.
But why do you want to do this when you can get the fitted probabilities
from predict()?

I hope this helps.
 John

--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox 
-------------------------------- 
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Tate Avery
> Sent: Friday, August 12, 2005 2:50 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Manually Calculating Odds from POLR Model
> 
> Hello,
> 
> I am using polr(...) to generate a model.  The summary shows 
> the coefficients and the intercepts.
> 
> For example:
> 
>     coefficient for x1 = c1
>     coefficient for x2 = c2
> 
>     intercept A|B = i1
>     intercept B|C = i2
> 
> I can then run predict(..., type="p") with the model and see 
> the odds for each factor.
> 
> For example:
> 
>       A        B        C
> 1    0.3     0.5      0.2
> 2    0.4     0.1      0.5
> 
> What I really want to be able to do is take the 2 
> coefficients, the 2 intercepts, the x1 & x2 values and 
> manually calculate the probabilities generated by predict().
> 
> I have been searching quite extensively for the underlying 
> calculations that transform the polr output and the input 
> variables into the final output odds.  I have tried a number 
> of dead-end roads so far.
> 
> So, if anyone has any information on how to do this or where 
> I can find out, I would be extremely grateful.
> 
> Thank you for your time,
> Tate Avery
> 
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