Incrementality
Incrementality is the revenue a marketing effort actually caused — the lift over what would have happened without it. Measured through geo holdouts, user-level holdouts, and synthetic control experiments.
Incrementality testing is the gold-standard answer to the question every performance marketer wants: 'how much did this channel actually grow the business?' Platform-reported conversions almost always overstate contribution because they count organic and brand traffic that would have converted anyway.
The two common incrementality designs: geo holdouts (turn off a channel in some geographic markets for a window, compare revenue against matched control geos) and user holdouts (Meta Conversion Lift, Google Uplift studies that hold some users out of your ads and compare conversions).
Incrementality usually reveals that last-click attribution overstates Meta, Google Search (on brand keywords), and retargeting. It often understates YouTube, Display, and top-of-funnel work. Results reshape budget allocation — which is why brands running mature performance marketing do at least quarterly incrementality studies.