A randomly selected group of eligible users who are deliberately not shown the campaign so their behavior can be compared with the exposed group. The difference in conversion rate between the two groups is the campaign's incremental effect. Platform-run holdouts use ghost ads (the auction is won but nothing is shown); older methods showed a public service ad instead.
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Cite this entry:
Agency Echelon. "Holdout." Agency Echelon Reference, reviewed September 2026. https://agencyechelon.com/reference/glossary/holdout/