Given that Amazon's system can be described as: "... for every item i1, we want every item i2 that was purchased with unusually high frequency by people who bou

Given that Amazon's system can be described as: "... for every item i1, we want every item i2 that was purchased with unusually high frequency by people who bought i1", I'd say it's completely rational that their system recommends you vacuum cleaners if you've already bought one.