comparing the performance of a score across different populations A very well validated score exists. This score predicts mortality after disease A when patients present from country W . The score is a 4 point score. 
If a subject presents from the community with disease A and his score is 1, mortality is x1. 
If a subject presents from the community with disease A and his score is 2, mortality is x2 etc...
I have a population with disease A. Some present from country W and some from country M. each subject has a clear classfication= mortality ie dead or alive. 
The score when applied to those from country W seems to perform as expected for each category. 
meaning individuals with score 1 seems to have a mortality very close to x1 and those with score 2 close to x2 etc...
However for population M, that is not the case. score 1 mortality is far from x1 and score 2 far from x2 etc.... 
1- is there a statistic that proves that for my sample population (from country W) the concordance between the expected mortality at each score category and mortality is very good?
2- is there a statistic that shows that for the sample from country M this is not the case ? 
In other words, for a validated score, how can I show that it expectedly performs well in the target population but not so in another. 
please be elaborate and simple. I am not a statistician. 
 A: I hope this points a way to analyzing your
data. If you don't think it is the right
approach for your data, please explain you
misgivings.
From what you say, it seems you can put counts of subjects into the 16 'cells' of
the table below. Then, in the Total column at the right you will put counts
of the total numbers of subjects from Countries
M vs, W, and Dead vs. Alive.
Notice that you will need counts not percentages or proportions. 
                   Score
             -----------------
Cntry  Mortl   1   2   3   4      Total
---------------------------------------
  M    Dead
       Alive
  W    Dead
       Alive
---------------------------------------
Total

This is a three-way contingency table. It contains counts for three
categorical variables:  Score (1,2,3,4), Country (M,W), and Mortality (Dead, Alive). 
The link gets you started with the analysis in R statistical software for a particular
dataset. That dataset has categorical variables Treated (Y,N), Sex (M, F),
and Improvement (None, Some, Marked).
