Why Doesn’t Amazon’s Price Sort Work?

Pretend you’re in the market for a new iPod. You go to Amazon and type in a search. Then, you sort the results by price, low-to-high. For infrequent Amazon users, the results may be surprising [1].

The results are far from monotonic. This is true regardless of whether Amazon’s sort considers only new items or if it also evaluates the price of used items from the “Amazon Marketplace”. The consensus among Amazon pros is that the site uses the latter approach. If that’s the case, only 11 of the 24 iPods on the first page are more expensive than the iPod that immediately precedes it in this rank of “increasing” price.

Why would Amazon fail badly at something that seems so simple? The immediate cause is probably caching. Amazon likely has a task that periodically caches the low price for each item. The inventory of used items in the Amazon Marketplace changes constantly and the...