You just need the data on who does (and by extension, doesn't) see the ad, as well as the shopping behavior of those same people. Both of which can be had by someone who isn't supplying the ad space or the exchange.
No, you misquote me. I said that is is impossible to observe incrementality. It is rather trivial to estimate incrementality, statistically, but all statistical models produce estimates. Not observations.
How do you get the data if you're not a supplier? You contract for the supplier.
