http://en.wikipedia.org/wiki/Doubling_time Article and mathematics
http://www.miniwebtool.com/doubling-time-calculator based on rate
http://www.doubling-time.com/compute.php based on two or more samples and intervals
http://www.google.com/search?aq=f&sourceid=chrome&ie=UTF-8&q=doubling+time+calculator Google Search
