Quantization of Books

In response to my post about Amazon sales-rank tracking, Matthew Beckler created just such a tracker:

That's the last few days' worth of hourly rankings of How to Teach Physics to Your Dog, as of 10am Eastern. Enormous dork that I am, I find this really fascinating, and not just in an absolute-number sense (because, really, these numbers don't mean much of anything).

The big thing that jumps out at me is the quantization of books. You see a bunch of sharp, downward-going jumps in the graph, which presumably correspond to discrete book purchase events. In between jumps, there's a slow upward trend, taking 6-8 hours to erase the jump from a single book (lower numbers are better, remember), which presumably says something about the rate at which Amazon's ranking algorithm depreciates recent purchases. The jump is also fairly consistently smaller when starting at lower numbers, which also makes sense-- the vast majority of books at Amazon haven't sold anything recently, so you can leapfrog a lot of them with a single sale, but once you enter the ranks of books that are currently selling copies, the relative effect of a single sale becomes much less.

And so on. I am inordinately fond of looking at graphs of data, even relatively meaningless data like this. Consider yourselves lucky that I don't have the data as a table of numbers, thus sparing you from curve fits...