A sequential trial is preferred to a fixed-size trial when the time interval between the administration of treatment and the outcome is prolonged.
In a randomized sequential design comparing two treatments, A and B, the patients receive both of the treatments in sequence, with some patients randomized to receive A before B and the remainder receiving B before A.
Sequential trials rely on systematic allocation of patients to treatments to ensure that patients are allocated to the treatments in a sequential manner.
Subgroup analysis is a form of sequential analysis in that the subgroups of interest are analysed in sequence after the whole group of individuals has been analysed.
The results of pre-planned interim analyses are generally not circulated to trial investigators, other than the Data Safety and Monitoring Committee, unless the trial is stopped.