Measurement
Incrementality
The share of results that would not have happened without the advertising, measured by holding out a group rather than by attribution.
What makes it wrong.
The mechanism, not the warning.
Retargeting scores best on attribution and often worst on incrementality, because it addresses people who were already going to buy. A channel can be the top performer in every attributed report and add nothing.
In detail.
Attribution asks which ad touched the customer. Incrementality asks whether the customer needed touching. The two answer different questions and the second is the one that decides whether a budget should exist at all.
It is measured with a holdout: a comparable group that is not shown the advertising, and whose behaviour becomes the baseline.