Work out attrition adjusted sample instantly with clear inputs, formula shown and shareable results.
Sample size must be inflated for both attrition and screening failure, and the two compound. Needing 200 at analysis with 20% attrition means enrolling 250, and a 15% screen failure rate means screening 295 - an inflation factor of 1.47. Planning only for attrition would leave the study 45 participants short.
Enrolment target
enrol = analysable target / (1 - attrition)
Screening target
screen = enrolment target / (1 - screen failure rate)
Intention-to-treat analyses everyone randomised, but missing outcome data still reduces effective power, so inflation is still needed.
It varies hugely: 5-10% for a short single-visit study, 20-30% for a year-long trial, and more for hard-to-reach populations.