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Holdout group
A group of people deliberately kept from seeing a campaign, so you have something to compare the exposed group against.
Example
10% of a campaign's eligible audience is randomly excluded from targeting entirely — their behaviour becomes the 'what would have happened anyway' baseline.
Why marketers care
Without a genuine holdout, there's no way to know how many conversions would have happened even without the ad — which is the entire basis of incrementality testing.