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Designing a trial: the question, the participants, and the allocation

A trial starts with a question specific enough to test, and PICO forces that: population, intervention, comparison, outcome. Free HPV vaccination against standard pricing, measured as uptake at six months, is ready for a trial. A loosely defined education session is not. Eligibility criteria then decide whose effect the trial estimates. The Ethiopian cholera vaccine trial excluded people with immunocompromising conditions, so on its own it can't tell us how the vaccine works for them. Allocation comes next: simple, block, stratified or cluster randomization, each trading simplicity for guaranteed balance.

Also covered: PICO · eligibility criteria · recruitment · block and stratified randomization