Incrementality

Incrementality measures the additional outcome caused by a marketing activity that would not have happened without it — the true causal effect, isolated from what would have occurred anyway.

This is the sharpest distinction in modern marketing measurement: attribution asks who gets credit; incrementality asks what actually changed because of the ad. A retargeting campaign can show a spectacular attributed ROAS by claiming credit for purchases from people who were already about to buy — incrementality testing is the method that catches this by comparing an exposed group against a genuinely comparable holdout group that saw no ads at all.

The standard method: split a comparable audience into a test group (exposed to ads) and a control group (held out), run the campaign, and measure the difference in conversion rate between the two groups. That difference is the incremental lift — the portion of results genuinely caused by the advertising, not just correlated with it.

Incrementality testing costs real, near-term revenue (the holdout group deliberately doesn't get ads that might have converted them), which is why most teams run it periodically — quarterly checks on major channels — rather than continuously on every campaign.