Clinical trials are often described as phase I, II, III, and sometimes phase IV. The number of participants varies greatly depending on the phase of investigation, but also depending on the cancer type, situation, and expected outcome. Phase I trials are principally charged with determining the safe and optimal dose of a new drug or drug combination. Such trials may involve 20 to 40 patients. Phase II trials are where the potential of a drug to be effective is first evaluated. Typical phase II studies range in size from 50 to 250 participants. The higher end of that range is more common in randomized phase II studies where 2 or more doses or drug combinations are being examined side by side. Phase III trials are the ultimate test of a new treatment. Participants are randomly assigned to the new treatment or the current standard. These trials involve hundreds and sometimes thousands of participants. The number of participants needed is determined by the question being asked, the expected magnitude of the difference between the two treatments, and the percentage of patients who are expected to experience the outcome being measured. For example, trials that seek to reduce the risk of cancer recurrence are often very large in part because many of the participants (thankfully) don’t suffer a recurrence at all. A large number of participants are therefore needed to make sure that enough of them have a recurrence so that a difference can be detected.
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