How can I compare the following mutual information values? How can I compare the following mutual information values ? I'm just wondering what's the most appropriate way to display them in my report table.
I'm computing them with this formula 
where e and c are clusters and the intersection is the number of elements in common.
For each couple e and c I have a I value (mutual information). Successively I average over all e belonging to the same category (not shown in the formula) and I end up with a table like:
cat1 0.0123
cat2 0.0012
cat3 0.0009
cat4 0.0100
...

The mutual dependency values are usually very low (around 0.01), because n (total amount of documents in the collection) is very high.
Should I use another measure, or... what do you suggest ?
thanks
 A: Are you after the mutual information between two clusterings? Marina Meila has introduced the 'variation of information' metric based on mutual information (see e.g. http://www.stat.washington.edu/mmp/Papers/icml05-compare-axioms.pdf). That would be quite appropriate to use. She also discusses alternative metric distances between clusterings. One of these (the split/join distance) is a bit more easily interpretable as the number of nodes that need rearranging between clusterings.
Alternatively, if you are not after a clustering-clustering comparison but more interested in individual events, you may consider using the hypergeometric P-value to consider the significance of intersection sizes between sets.
A: Mutual information measures the independence between two random variables and is maximal when these random variables covary [T. M. Mitchell, Machine Learning. McGraw-Hill Science/Engineering/Math](Available: http://www.worldcat.org/isbn/0070428077).
I am however not sure whether I understand your question correctly.  Is n the total number of clusters? (You used the term number of documents).
If there are many clusters and items that you are comparing do not covary in many/most of them, then mutual information will be low in most of them and by using the mean (average) you might consequently get small values as results.
Possibly changing the way that this problem is represented or using a different measure might allow you to analyse your data phenomenon better. 
