Incrementality
Incrementality is the share of conversions that advertising actually caused — conversions that would not have happened without the ads, as opposed to conversions the ads merely touched.
Attribution asks "which ad touched this conversion?"; incrementality asks "what would have happened anyway?" The gap between them is largest exactly where reported performance looks best: brand search, retargeting, and any campaign harvesting people who had already decided.
The workable tests do not require a data team: geo holdouts (turn a channel off in matched regions), audience holdouts, and pause-tests on suspiciously efficient campaigns. Run them when a budget line becomes big enough that being wrong about it matters.
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