# Mutual Information for clustering

I'm working on a document clustering application and decided to use Normalized Mutual Information as one of the measures of effectivenes. But I don't really understand how to implement this in that situation. In http://nlp.stanford.edu/IR-book/html/htmledition/evaluation-of-clustering-1.html the the formula is transformed to (185), and in this publication (www.shi-zhong.com/papers/comptext2.pdf‎, page 8, formula 17) it looks slightly different, n(h,l) is not divided by total number of documents N. So, which formula is correct? I would be very grateful for possibly simple explantation.