R help - Sep 2004 - Measures of association for ordinal data

In a classification problem with ordinal data (classes 1 - 4), I used 
multidimensional optimization to maximize gamma (Goodman's measure of 
association) between observations and predictions. This resulted in the 
following frequency table (rows = observations, columns = predictions):

	     1         2        3        4      sum
   1      16993    1    4854    27    21875
   2       1308     0    1691   120    3119
   3       1427     1    4587   434    6449
   4        289      0    1965   593    2847
sum   20017   2   13097  1174

Predictions for class 1 are fairly good, however, classes 2 and 4 are 
underrepresented and class 3 is overrepresented, as shown by the 
marginal sums.
Is there any measure of association implemented in R that takes into 
account the prevalence of certain classes?

Thanks for your help!

Christof