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Recruitment duration depends on the screening funnel, not just the target. Needing 240 participants with a 25% screen failure rate means screening 320, and at twelve screened a week that is 26.7 weeks - just over six months, at an effective enrolment rate of nine a week. Underestimating the failure rate is the classic cause of trial delay.
Participants to screen
screened = target / (1 - screen failure rate)
Duration
weeks = participants to screen / screening rate per week
Because pilot rates come from enthusiastic early sites. Recruitment typically slows once the easiest eligible participants are exhausted.
Yes. Recruiting 10-15% above target protects the analysis against withdrawals and protocol deviations.