Clinical trials are generally tests of the effectiveness of medical and surgical interventions. With pharmaceuticals, trials go through various phases, identifying safety and efficacy beyond the preclinical trial stage. Trials use samples of patients drawn from a relevant population of patients or people at risk and many are multi-centred and international in nature. They vary greatly in their use of controls (for example, some compare the procedure being studied with a placebo, others with a common practical alternative) and in other aspects of their design, their size, duration, choice of outcome, endpoint. See Phases in Clinical Trials.
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