I am working on a research project in which I am using inverted index for terms and documents in one of the techniques. I am doing algorithmic analysis for all the methods so that I can compare and contrast them on asymptotic time bounds. For inverted indexes, I need to find out the average number of documents containing any term. I have looked at the relevant literature but could not find anything relevant. Heap's Law is present but it tells us something else.
Do you have some idea on how can I get this value, apart from doing empirical evaluation on some datasets? Or if someone has done the algorithmic analysis for inverted indexes before that I can cite.