If you're under pressure to grow revenue, our ROAS looks good isn't proof. It's a story that shows correlation, not causal impact. Incrementality testing answers the real question: did we create revenue that wouldn't have happened anyway?
That's what an incrementality holdout test is for. It gives you a clean counterfactual — a control group that did not get the treatment — so you can measure incremental revenue instead of credited revenue.
I'll walk you through when holdouts beat classic A/B testing, how to design them so Finance won't roll their eyes, and how I turn results into a decision I can defend.