What is the "pdm" stat in the "rms" R package? 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?
 A: I don't know yet about the background of the statistic but below is an illustration how it is computed.
$$pdm = \frac{1}{n} \sum_{k=1}^n \left| \hat{P}(Y \geq median|X_k) - 0.5 \right| $$
It is an indication of how much the conditional predicted probability varies around the point of the marginal median.
library(rms)

###
###  generate some data according to a logit model
###
set.seed(1)
n = 10^2
k = 5
x = runif(n,-2,2)  # predictor
noise = rnorm(n,0,1)   # noise
y = x + noise        # latent variable
bounds = seq(-2,2,1) # values to be predicted
z = as.numeric(sapply(y, FUN = function(yi) sum(bounds<yi)))  # ordinal variable

### compute ordinal regression
mod = orm(z ~ x, family =probit)


### marginal median of the data
median = mod$stats[3]
### predictions hat p(y>=median | x) for all x in the data
pr = coef(mod)[median] + coef(mod)[k+1]*x
prs = pnorm(pr)
### mean absolute deviation  ... 0.3308655
mean(abs(prs-0.5))
### value from the function  ... 0.3308655
mod$stats[14]

A: 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.
