Incrementality testing is a geo or audience holdout experiment that measures how many conversions happen only because of marketing spend (vs. organic), answering the true "what did the ads actually cause" question.
Incrementality testing runs a controlled experiment — typically a geographic split or audience holdout — where one group sees ads and a matched group does not. The difference in conversion rates is the true incremental lift caused by the advertising. This corrects for clients who would have come anyway via referral or brand search. For UK IFAs scaling spend past £15k/month, incrementality testing is the only reliable way to confirm marketing efficiency.