Attribution
Incremental ROAS
The true return on ad spend after removing conversions that would have happened without any advertising. Measured through holdout testing, geo-lift studies, or conversion lift experiments. Always lower than platform-reported ROAS because platforms count organic conversions as ad-driven. For example, if Meta reports a 5x ROAS but 30% of those conversions were organic, your iROAS is actually 3.5x. This metric is the single most important number for budget allocation decisions because it tells you the real incremental value of each dollar spent.
Why it matters
It is the only version of ROAS that answers the question a budget decision actually asks: what would have happened if I had not run this. Reported ROAS cannot answer it because it never observes the counterfactual.
In practice
Establish it with a holdout or geo test rather than a model. Any figure derived from the same attribution data that produced reported ROAS inherits its bias.
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