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Types and Phases of Clinical Trials


  • Observational Studies: Behavioral research that involves people without any intervention or involves intervention in or manipulation of the subject’s environment and observes the subject’s reactions.
  • Record reviews and historical studies: Studies using existing public surveys, questionnaires and interviews as a base to develop an interview instrument which is first pre-tested on a small group of people from the subject population to identify and eliminate any biases or ambiguities in the way the questions are stated.
  • Epidemiologic Studies: Studies in which the investigator attempts to identify risk factors for particular diseases, conditions, or behaviors, or risks that result from particular causes, such as the environment or industrial agents.
  • Case-control studies: People with specific conditions (cases) and people without those conditions (controls) are selected to participate in the same study.  The proportion of cases and controls with certain characteristics are then compared.
  • Longitudinal studies: One or more subject cohorts are followed over a long period of time.
  • Controlled clinical trials: Prospective studies designed to assess the safety and efficacy of new drugs, devices, treatments or preventive measures in humans by comparing two or more interventions or regimens.


Each phase of a clinical trial has a different purpose. 

  • Phase I trials: Researchers test a new drug or treatment in a small group of people (20-80) for the first time to evaluate its safety, determine a safe dosage range, and identify side effects.
  • Phase II trials: The study drug or treatment is given to a larger group of people (100-300) to see if it is effective and to further evaluate its safety.
  • Phase III trials: The study drug or treatment is given to large groups of people (1,000-3,000) to confirm its effectiveness, monitor side effects, compare it to commonly used treatments, and collect information that will allow the drug or treatment to be used safely.
  • Phase IV trials: Post marketing studies delineate additional information including the drug's risks, benefits, and optimal use.


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