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The same measurement at different weeks can answer different scientific questions. Selecting the primary assessment solely for scheduling convenience may create a design inconsistent with the expected development or persistence of an effect.
Define the question and assumptions
Consider clinical meaning, measurement properties and expected loss to follow-up together. Clarify whether the objective concerns one visit or experience over a period. The statistical method should then match that question; an attractive model cannot supply the scientific rationale for choosing the assessment time.
Hypothetical study example
In a hypothetical trial, an early measurement captures rapid change while a later measurement addresses persistence. The team chooses its priority question rather than presenting these as interchangeable endpoints. Follow-up arrangements and sample size assumptions are reviewed against the information needed at that selected time.
Design review checks
- Explain how the assessment time connects to the intended clinical question.
- Evaluate what the early and late edges of the visit window imply.
- Consider treatment discontinuation and loss to follow-up before the chosen assessment.
Record the decision and its limits
The decision record should explain why the time is primary and how other assessments contribute. Choosing the most favourable visit after results are available undermines confirmatory interpretation. Other prospectively planned assessments should be reported in their appropriate secondary or supportive roles with relevant limitations.
Frequently asked questions
- Is a later assessment always more meaningful?
- No. The question and validity of the measurement determine its value.
- Can all visits be primary?
- Only with an explicit scientific rationale and a planned strategy for multiple evaluations.