In the following post, Dr. David Clarke brought up a point that I wanted to respond to: http://www.tmswiki.org/forum/threads/amazon-sales-data-for-books-about-tms.5091/#post-30960 My response gets pretty technical and if people reply it could get even more technical, so I thought it would be a good idea to start a new thread for the new discussion. Anyway, here is what Dr. Clarke wrote: I haven't had a chance to read up on this, but I wonder if what you are saying is captured in the idea that book sales follow a power law. The following paper provides an introduction to this idea: http://web.sfc.keio.ac.jp/~iba/papers/2008JJNAMS08-market.pdf The authors write, "In this paper, we analyze the real sales data of the book sales market in Japan. The data which we analyze is the data taken from POS (Point Of Sales) system of over 2,000 bookstores in all areas of Japan. The target term is from April 2005 to March 2006. As a result of analysis, it became clear that the relation between sales volume and sales rank follows power law in both of annual and monthly sales." In Appendix B of this article, economists use data to estimate a power-law formula that converts Amazon sales rank into weekly sales numbers: http://www.nimblebooks.com/wordpress/2006/06/power-law-converting-amazon-sales-ranks-to-units-sold/ I believe the resulting formula is Sales/week = e to the power (10.526-(.87*sales rank) See also this link. Practically speaking, it seems inevitable that the formula will only provide a very rough estimate given how complex the market is, how the market can change over time, and how very rapidly Amazon sales rank fluctuates for individual books. More reference can be found on Google. But for anyone reading this thread later on, the bottom line seems to be that books with low Amazon sales ranks sell a LOT of books, so it gets harder and hard to move up the ranks, the farther you go.