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st: nonsense category in ordered logit

From   "E. Michael Foster" <>
Subject   st: nonsense category in ordered logit
Date   Mon, 21 Feb 2005 15:33:41 -0500

I have data on an ordered outcome coded from 1 to 3. Suppose I suspect that the distinction between levels 2 and 3 is meaningless--i.e., nothing substantive would be lost if I just recoded the 3s to 2s.

One implication would be that the proportional odds assumption wouldn't not hold for the 3-category model because the difference between levels 2 and 3 has nothing to do with the X variables.

I could test this, but it seems like a weak test. The proportional odds assumption could be violated for any of several reasons.

How do I demonstrate to my psychologist colleagues (the measure is a psych measure) that collapsing 3 categories into 2 is harmless?

Michael Foster
Professor of Health Policy and of Demography

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