Geo holdout test
A geo holdout test pauses (or launches) advertising in a matched set of geographic regions while keeping it live elsewhere, then compares outcomes to measure the channel’s true incremental effect.
Geo holdouts are the most accessible form of incrementality testing: no user-level data, no privacy complications, no data-science team — just matched markets, a clean intervention, and enough time for the difference to exceed noise.
Design essentials: match regions on baseline sales trajectory (not just size), run through at least one full purchase cycle, and pre-commit the decision rule so results cannot be argued away after the fact.
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