Amazon's book rankings are puzzling

Do you ever pay attention to the book rankings on Amazon? I didn't used to, until I had books for sale. But now I'm befuddled by how they do their rankings. One would think that the more books are sold, the higher the book would rank on the bestseller list. But that doesn't appear to be the case, at least not entirely.

Case in point: My first nonfiction book (OS/2 Power User's Reference: From OS/2 2.0 Through Warp) was published in 1995, before Amazon existed. Before the book went out of print, it was added to Amazon's sales list (in 1997/1998), and sold a few dozen copies. Fast forward to 2007, and my first novel, The Mars Imperative, debuts. When the first copy sold, it rocketed up the sales rankings to #291,000+. 8^} Just out of curiosity, I looked up my old OS/2 book, and saw that it was ranked #4,100,000+. That made no sense. How could a book with dozens of sales be ranked 4 million places lower than a book with only one sale?

And it gets weirder. By the time the second (and last) copy Amazon had in stock sold, the book had dropped to #600,000+. Upon the second sale, it shot up to #78,777. Did that mean there were more than 500,000 books with exactly one sale to their credit? That hardly seemed likely. But how could one sale make a difference of 521,000+ positions otherwise?

Then, while waiting for the next shipment of nine books to arrive at Amazon, I noticed that the ranking had dropped to #700,000+. Did this mean that all those books with one sale had suddenly shot up to three or more copies? That seems unlikely. Perhaps those other books were all brand new and had just caught on and were now up to eight or ten copies sold; but come on, how likely is it that half a million books were just recently added to Amazon's sales list? And if they were indeed older titles that for whatever reason sold only one copy before, why would they all suddenly get a few more sales? A few books, maybe. But all half-a-million of them?

None of this makes sense (and still doesn't explain why a book with one sale would be ranked four million places higher than one with dozens of sales. The only answer I can think of is that time is factored in with the number of sales, such that a book with hundreds or thousands of sales from ten years ago is ranked lower than one with tens of sales today, especially if the older book is out of print and can't be ordered anymore. (I concede that there's no point in ranking a book highly that's out of print and can't be ordered. But in that case, why rank it at all?) Still, it's not exactly an accurate measurement of where a book ranks in terms of overall sales.

Well, I guess it's all pretty meaningless until a book reaches the top 40 or so and makes it onto the various bestseller lists.

That's something to look forward to, anyway. 8^}

(Update: Three more sales, and the ranking jumped to #93,727.)

Mark.