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Do the results apply to a different population entirely?

Transportability asks whether results apply to a population your sample was never part of, and no amount of better sampling fixes it — the people you want to talk about were never eligible. The move is to re-express what you found against the new population's composition. A 1997 HIV trial carries to a later population by taking its age-specific effects and reweighting them to that population's age structure. This works only when you know what modifies the effect and can measure it in both places.

Also covered: transportability · standardization · reweighting · effect modification