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A calendar date for interim analysis may not coincide with the amount of information needed for the planned decision. Enrolment counts, completed assessments and event counts are different indicators; define which quantity the design actually requires before setting operational expectations.
Define the question and assumptions
Connect the review trigger to the planned statistical method. In an event-driven study, completed recruitment may not indicate sufficient accumulated information. Delayed entry, event confirmation and follow-up affect the trigger. Specify how information is calculated and updated without allowing emerging treatment comparisons to determine convenient timing decisions.
Hypothetical study example
In a hypothetical trial, the scheduled review date arrives with fewer confirmed events than expected. The team checks the prespecified trigger instead of proceeding mechanically. Authorised statistical personnel assess consequences for decision boundaries and operational preparation, distinguishing genuine information shortage from administrative delays in confirming existing observations.
Design review checks
- Record the information measure, its definition and the authoritative data source.
- Show pending events separately from confirmed events available for analysis.
- Keep trigger checks and the actual analysis timing traceable throughout conduct.
Record the decision and its limits
An accepted plan contains workable rules for information arriving earlier or later than expected. A specialist assesses how actual information relates to interim boundaries. Calendar pressure does not justify silently changing design assumptions. Report the realised trigger and deviations alongside their consequences for interpreting the resulting decision.
Frequently asked questions
- Does half the enrolment mean half the information?
- Not in every design; endpoints and follow-up determine the relevant information.
- Do entry delays matter?
- Yes. They can affect the maturity and accuracy of information supporting an interim decision.