Metrics
Incrementality
The portion of measured conversions actually caused by the ad.
Incrementality is the gold-standard metric: of the conversions reported, what percentage would not have happened without the ad? Measured via geo holdout tests, ghost ads, or pre-post panels. Most direct-response ad platforms over-report conversions because they count conversions that would have happened organically. Brand-aware advertisers run quarterly incrementality tests to recalibrate ROAS targets — often discovering the platform-reported ROAS is 2–3× the incremental ROAS.
Example
DTC brand runs a 3-week geo-holdout on Meta in 8 mid-size US DMAs: revenue in held-out DMAs falls 11%, in active DMAs holds flat → incremental ROAS = 1.4× vs platform-reported 3.2×.