Sequential Analysis:
In sequential analysis, decisions about sample size and the type of data to be collected are made and modified as the study proceeds, incorporating information learned at earlier stages. One major application of sequential analysis is in clinical trials in medicine, where successful therapies can often be accepted by regulators sooner when their clinical trials follow a flexible sequential design. Such designs allow information to be "harvested" earlier than with a conventional fixed sample-size design, where conclusions must await the accrual of a fixed number of outcomes, specified in advance.
See Group Sequential Methods With Applications to Clinical Trials by Jennison and Turnbull