text: Clinical trials are classified by the research objective created by the investigators.
  * In an observational study, the investigators observe the subjects and measure their outcomes. The researchers do not actively manage the study.
  * In an interventional study, the investigators give the research subjects an experimental drug, surgical procedure, use of a medical device, diagnostic or other intervention to compare the treated subjects with those receiving no treatment or the standard treatment. Then the researchers assess how the subjects' health changes.
  Trials are classified by their purpose. After approval for human research is granted to the trial sponsor, the U.S. Food and Drug Administration (FDA) organizes and monitors the results of trials according to type:
  * Prevention trials look for ways to prevent disease in people who have never had the disease or to prevent a disease from returning. These approaches may include drugs, vitamins or other micronutrients, vaccines, or lifestyle changes.
  * Screening trials test for ways to identify certain diseases or health conditions.
  * Diagnostic trials are conducted to find better tests or procedures for diagnosing a particular disease or condition.
  * Treatment trials test experimental drugs, new combinations of drugs, or new approaches to surgery or radiation therapy.
  * Quality of life trials (supportive care trials) evaluate how to improve comfort and quality of care for people with a chronic illness.
  * Genetic trials are conducted to assess the prediction accuracy of genetic disorders making a person more or less likely to develop a disease.
  * Epidemiological trials have the goal of identifying the general causes, patterns or control of diseases in large numbers of people.
  * Compassionate use trials or expanded access trials provide partially tested, unapproved therapeutics to a small number of patients who have no other realistic options. Usually, this involves a disease for which no effective therapy has been approved, or a patient who has already failed all standard treatments and whose health is too compromised to qualify for participation in randomized clinical trials. Usually, case-by-case approval must be granted by both the FDA and the pharmaceutical company for such exceptions.
  * Fixed trials consider existing data only during the trial's design, do not modify the trial after it begins, and do not assess the results until the study is completed.
  * Adaptive clinical trials use existing data to design the trial, and then use interim results to modify the trial as it proceeds. Modifications include dosage, sample size, drug undergoing trial, patient selection criteria and "cocktail" mix. Adaptive trials often employ a Bayesian experimental design to assess the trial's progress. In some cases, trials have become an ongoing process that regularly adds and drops therapies and patient groups as more information is gained. The aim is to more quickly identify drugs that have a therapeutic effect and to zero in on patient populations for whom the drug is appropriate.
  Clinical trials are conducted typically in four phases, with each phase using different numbers of subjects and having a different purpose to construct focus on identifying a specific effect.