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I am building an ordinal logistic regression model (ORM). In order to fit my ORM model, I am using the 'orm' function of 'rms' package from R (http://cran.r-project.org/web/packages/rms/rms.pdf).

Now I am trying to assess the goodness of fit of my model. By reading the R documentation, I can see the following statement in the 'stat' property of the 'orm' object (p. 98):

(...) Nagelkerke $R^2$ index, the $g$-index, $gr$ (the $g$-index on the odds ratio scale), and pdm (the mean absolute difference between 0.5 and the predicted probability that $Y\geq q$ the marginal median) (...).

I don't have enough background to understand the short description of the pdm measure. But when I try to do more research on this measure, I am not able to find related material (e.g. I've been finding "prescription drug misuse"). In summary, my question is:

Would you know if the 'pdm' measure has some synonym which is more widely used? Or can you provide some references where I can study the pdm metric?

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This link: https://www.rdocumentation.org/packages/rms/versions/5.1-0/topics/validate.lrm describes pdm as a "new" metric, which would imply that is has not been used before.

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  • $\begingroup$ If this is new why would a question about it have come up in 2014? $\endgroup$ – Michael R. Chernick May 10 '17 at 13:36
  • $\begingroup$ I meant new in the sense of not previously reported in the literature, or used outside of the rms package. I myself am not sure, I am just going on based on what they are saying in the package it sounds like this is a new metric they came up with, as opposed to a standard metric that they gave a different name to. $\endgroup$ – Josh May 10 '17 at 13:43

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