Sample Size
Planning for Participant Attrition: Correct Inflation and Missing-Data Strategy
Incorporate attrition using the correct formula, timing, causes and analysis strategy rather than adding a percentage mechanically.
Admin
June 20, 2026
8 min read
Attrition inflation is not simple addition
If n completers are required and attrition is d, recruit n/(1-d). Multiplying by 1.20 for 20% attrition is not exact.
Practical framework
Use comparable follow-up evidence.
Consider differential attrition.
Separate dropout from intermittent missingness.
Plan appropriate sensitivity analyses.
Reporting
Report expected rate, evidence, formula, rounding and final target.
Common mistakes
Adding the percentage directly.
Using an unsupported rate.
Assuming ITT automatically solves missingness.
References
Accessed 20 June 2026. General methodological information.