Re: st: Help please re Ocratio

["Jon Heron (ALSPAC)" <Jon.Heron@bristol.ac.uk>]

I could use an extra bit of clarification:


It sounds like, if I want to fit an ocratio model and then examine the
individual pairs of effects I either fit two logistics

   1 [=1] versus 2 & 3[=0]
   2 [=1] versus 3 [=0]

and compare with an ocratio on an outcome ordered 1,2,3


or fit two logits

   1 [=0] versus 2 & 3[=1]
   2 [=0] versus 3 [=1]

and compare with an ocratio with outcome ordered the other way i.e. 3,2,1


If this is the case, and a quick experiment with Stata suggests it is,
and since the continuation ratio model is not reversible (unlike
proportional odds), how do I decide which one to do?

I guess the former




Jon
--------------------------------------------------
Dr Jon Heron
Statistics Team Leader
ALSPAC, Dept of Social Medicine
24 Tyndall Avenue
Bristol BS8 1TQ
Tel: 0117 3311616
Fax: 0117 3311704





--On 30 October 2007 08:50 -0400 Anders Alexandersson 
<andersalex@gmail.com> wrote:


> Please provide complete references. Rory Wolfe's command -ocratio-
> uses a "forward stopping" continuation ratio model, P[Y=j|Y>=j] (i.e.,
> like "Armstrong and Sloan" below), but with reverse sign of the slope
> compared with logistic regression on expanded data so that the sign of
> the slope is the same as in the usual proportional odds model.
>
> Anders Alexandersson
> andersalex@gmail.com
>
> Jon Heron (ALSPAC) <Jon.Heron@bristol.ac.uk> wrote:
> >  I would like to fit a continuation ratio model to a 6-level outcome
> >  using ocratio. I would then like to assess the constancy of model
> >  parameters by fitting a series of logistic models
> >
> >  A quick lit search is showing variability in which comparisons are being
> >  made with a CR model.
> >
> >   e.g. for 3-level outcome - 1/2/3
> >
> >   Armstrong and Sloan (1989)
> >     1 versus 2 & 3
> >     2 versus 3
> >
> >   Manor, Matthews and Power (2000)
> >     1 versus 2
> >     1 & 2 versus 3
> >
> >   Greenland (1994)
> >     2 & 3 versus 1,2 & 3
> >     3 versus 2 & 3
> >
> >  I imagine these are all permitted alternatives, but i would like to know
> >  which particular comparisons are made using ocratio so I can fit the
> >  appropriate logistic models.
> *
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> *   http://www.ats.ucla.edu/stat/stata/



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