Sample Size
The Target Effect Behind Sample Size: From Clinical Rationale to Assumption
Choose a target effect using clinical importance, external evidence and sensitivity scenarios rather than automatic Cohen thresholds.
Admin
June 20, 2026
8 min read
Effect size is the most consequential sample-size input
An optimistic target underpowers the study; an unnecessarily tiny target can make it infeasible. Justify the value prospectively.
Practical framework
Define the primary outcome and meaningful target.
Use intervals from comparable populations and designs.
Treat small pilot estimates as uncertain.
Calculate optimistic, expected and conservative scenarios.
Reporting
Report the target difference, variance, standardisation and evidence source.
Common mistakes
Using Cohen labels without context.
Treating a pilot estimate as precise.
Selecting evidence only from significant studies.
References
Accessed 20 June 2026. General methodological information.