[bschne]

Well that works in some ways, but it can also be very hard to disentangle the actual causal impact of an ad campaign! Absent running an experiment, you don't really know if people would or would not have bought through some other channel without your campaign.

The most striking example is probably the experiment Tadelis convinced eBay to run on their brand-name ads, though the subtlety applies to less obvious cases of questionable ad spend too — https://faculty.haas.berkeley.edu/stadelis/Tadelis.pdf

[tetsusaiga]

I'll just reference my other replies to the first part here, but thank you for that link too. I'll have to give it a read when I have more time later today