Stanford's Sam Savage has written an excellent book on the topic of disaggregating average data and mistakes made from using averages. Here's a summary of the book on his website: http://www.stanford.edu/~savage/flaw/

and here's a link to his book, titled The Flaw of Averages: http://www.amazon.com/Flaw-Averages-Underestimate-Risk-Uncer...