[auntienomen]

There are two significant complications:

1) Google is running many parallel ad campaigns, which may target the same individuals. This in some ways gives opportunities, because one can run 'natural experiments' on the effeciveness of advertising for X by simply selecting the people who never saw the ad for X. But there is also probably some legal peril; Google has to be careful about what promises it makes to people purchasing ads.

2) Google has very little incentive to release the results of any such studies, because -- whether or not advertising works -- they don't need their customers to have accurate side-info about the value of advertising.

[initplus]

They have every incentive to release the results - if they showed that the market was undervaluing advertising.