Incrementality.

Incrementality is the share of measured conversions that would not have happened without the ad. Attribution assigns credit; incrementality asks whether the ad caused anything. It is measured with holdouts or geo experiments, and it is the defense against paying to reach people who were converting anyway.

A retargeting campaign reports stellar ROAS. A geo holdout withholds it from matched regions, and conversion barely differs: most credited conversions were coming regardless. The budget moves to prospecting, where the same test shows real lift, and the account earns more with the identical total spend.