[soneil]

If not showing repeat vacuums does not improve clicks, I'd be very curious if it reduces clicks either.

It seems like what you posit is only a mystery if we take it on faith that targeting works. Once that faith is lost, you can also approach it from the angle that if showing them something we think they want is just as effective as something we know they don't want, you're actually proving that "what we think they want" isn't working either.

I mean, given "we know you've bought a vacuum cleaner" and "we think you'd like a kettle" - if ads for either are equally effective, either there's a lot more people collecting vacuums than either of us would have expected - or we're entirely wrong about the kettle. And it seems to me that our blind faith in targeted advertising leads us to wonder why people want so many vacuums.

[jefftk]

I don't think it's a mystery. Ad targeting usually doesn't know which people have just bought a vacuum, just that someone has been looking at vacuums recently. People who have been looking at vacuums are far more likely than the typical person to buy one, so it's not surprising that it would be worth showing them an ad reminding them that they can buy one from you. Especially with higher margin products like mattresses or cars.

But how do we explain how there are also advertisers like Amazon who do know that you just bought a vacuum from them and still show you more ads for it? Since Amazon is in a position to run principled A/B tests on whether showing these ads leads to sales that otherwise wouldn't have happened, and they are the kind of organization I'd expect to get this right, this part I am willing to accept without external evidence. It's probably that the likelihood of additional purchases of the same item, for yourself or others, is high enough, combined with that the cost of advertising to you is low enough.