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2. Selection Bias: Concealment


This question investigates the likelihood that the group to which a subject was assigned could be predicted by the person enrolling the individuals.

Those enrolling patients are aware of the group (or period in a crossover trial) to which an enrolled patient will be allocated (major problem in ‘‘pseudo’’ or ‘‘quasi’’ randomized trials with allocation by day of week chart number, etc).

Signalling Questions

  • Was the allocation sequence fully concealed until participants were enrolled and assigned to interventions?
  • Could the investigators enrolling patients have guessed which group a participant would be assigned to?


Example of appropriate concealment: The use of sealed envelopes or computerized random group assignment at the time of enrollment of a patient.

Examples of inappropriate concealment: Block randomization that would allow the person enrolling subjects to know to which group the next patient enrolled would be assigned.