Regarding precision and recall for the highly unbalanced validation data set 
Possible Duplicate:
Optimising for Precision-Recall curves under class imbalance 

I built a classification model and tested it against a validation data set. The positive set is composed of 86 cases and the negative set is composed of 1256 cases. The confusion matrix is as follows
                     True positive  True negative   precision
   Predict positive    55               338          13.99%
   Predict negative    31               918          96.73%
           Recall      63.95%       73.09%  

The precision and recall for this classifier is not good, especially for the positive precision. However, the negative cases are much more than the positive case. I am not quite sure that,  for  this kind of unbalanced data, can we still use the precision and recall as the performance evaluation as usual?
 A: I work in biomedical text classification, where this sort of situation happens all the time. You're exactly right--precision and recall aren't all that informative for highly-skewed data. I tend to use AUC as my performance metric, as it's not sensitive to class distribution. 
A: You could introduce a cost function, consistent with your application,  with values for TP, FP, TN, FN  and optimise your predictors for that.
A: I think you need to be clearer what you mean when you say "Not valid": in the sense that they summarise the contingency table, they are valid, but they are biased in the case of highly imbalanced data. One alternate measure you can look at, which tends to be more stable across class balance, is the mean of true positive rate and (1 - false positive rate). 
You should be careful about what you want to do with this, though: precision on your positive class is a useful metric to have, because optimising that recall/precision tradeoff on an infrequently occurring class is often the goal of the practical application of classifiers.
