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Block randomisation can help control arm sizes, but small known blocks may make future assignments easier to predict. The design decision should consider the operational relationship between balance and concealment.
Define the question and assumptions
Define who needs access to block information and why. Separate sequence generation, system configuration and site use into distinct responsibilities. Variable block sizes need a statistical rationale and implementation testing; merely describing a process as random does not demonstrate adequate protection against predictable allocation.
Hypothetical study example
At a hypothetical site, an employee says previous assignments allow prediction for every fourth participant. The team examines enrolment order and information access. Rather than closing the issue solely by enlarging blocks, it also reviews system outputs and communication between operational personnel.
Design review checks
- Check which protocol and operational documents disclose block details to site users.
- Validate the production sequence without unnecessarily disclosing its structure to sites.
- Avoid treating an unexpected allocation pattern as proof of misconduct by itself.
Record the decision and its limits
The acceptance record explains the method, access restrictions and testing evidence. A visible sequence pattern can also occur by chance; assessment should address the process and documented events. Any change during an ongoing study needs a separate assessment of consequences and the applicable review pathway.
Frequently asked questions
- Are larger blocks always safer?
- No. Information access and the operational workflow must be considered together.
- Does an observed pattern prove an error?
- No. Random sequences can contain patterns; independent process review is needed.