The Amazon Percentage Trick

Well, as the saying goes:

>> Facts are stubborn things, but statistics are more pliable.

But to [maybe not your main] point, sales rank and quantity sold are most definitely HIGHLY correlated (if not only for the simple fact that sales rank is endogenously determined by quantity sold, and other variables, in Amazon's algorithms). In fact, the log(sales_rank - 1) for items in most categories on amazon most likely follows an exponential distribution. In this case, if you're able to put together an estimate for the distribution's shape parameter, then in theory you can use it and the sales rank to back-out an estimate for the actual quantity sold.

FWIW.

This is a bit of a UX Dark Pattern [1], isn't it? The percentage is not strictly lying, it just means something else than you'd expect.

[1] http://darkpatterns.org