Incrementality
Definition
Incrementality is the additional revenue an ad genuinely caused, as opposed to revenue that would have occurred anyway. It is measured by withholding advertising from a comparable group.
Attribution answers 'which touchpoint gets credit'. Incrementality answers the only question that matters commercially: 'would this sale have happened without the ad'. Branded search and retargeting are the classic offenders — both report excellent ROAS while largely intercepting demand that already existed.
The practical test is a geo holdout: suspend spend in one comparable region, hold another constant, and compare revenue over a fixed window. It is the closest thing to a controlled experiment available to most marketing teams, and it routinely changes budget allocation more than a year of dashboard analysis.
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