By HFA Staff with sourcing from the National Institutes of Health鈥檚 National Library of Medicine

A clinical study involves research using human volunteers (also called participants) that is intended to add to medical knowledge. There are two main types of clinical studies: clinical trials (also called interventional studies) and observational studies.

In a clinical trial, participants receive specific interventions according to the research plan or protocol created by the investigators. These interventions may be medical products, such as drugs or devices; procedures; or changes to participants’ behavior, such as diet. Clinical trialsmay compare a new medical approach to a standard one that is already available, to a placebo that contains no active ingredients or to no intervention. Some clinical trials compare interventions that are already available to each other. When a new product or approach is being studied, it is not usually known whether it will be helpful, harmful or no different than available alternatives (including no intervention). The investigators try to determine the safety and efficacy of the intervention by measuring certain outcomes in the participants. Clinical trials used in drug development are often described by phase. These phases are defined by the Food and Drug Administration.

In an observational study, investigators assess health outcomes in groups of participants according to a research plan or protocol. Participants may receive interventions (which can include medical products such as drugs or devices) or procedures as part of their routine medical care, but participants are not assigned to specific interventions by the investigator (as in a clinical trial). For example, investigators may observe a group of older adults to learn more about the effects of different lifestyles on cardiac health. Investigators assess health outcomes in groups of participants who receive interventions, but participants are not assigned to specific interventions by the investigator as they would be in a clinical trial.

Who Conducts?

Every clinical study is led by a principal investigator, who is often a medical doctor. Clinical studies also have a research team that may include doctors, nurses, social workers and other health care professionals. Clinical studies can be sponsored, or funded, by pharmaceutical companies, academic medical centers, voluntary groups and other organizations, in addition to federal agencies. Doctors, other health care providers and other individuals can also sponsor clinical research.

Who Participates?

Some studies seek participants who have the illnesses or conditions that will be studied while other studies are looking for healthy participants. Some studies are limited to a predetermined group of people who are asked by researchers to enroll.

Who’s Eligible?

The factors that allow someone to participate in a clinical study are called inclusion criteria. The factors that disqualify someone from participating are called exclusion criteria. Criteria are listed in the study protocol. Factors are based on characteristics such as age, gender, the type and stage of a disease, previous treatment history and other medical conditions.


Studies take place in hospitals, universities, doctors鈥 offices and community clinics, depending on who is conducting the study.


The length of a clinical study varies, depending on what is being studied. Participants are told how long the study will last before they enroll.


Informed consent is a process used by researchers to provide potential and enrolled participants with information about a clinical study. It protects participants and provides enough information for a person to understand the risks of potential benefits of and alternatives to the study. (See this blog post for more information on informed consent.)

Institutional Review Boards

Each federally supported or conducted clinical study and each study of a drug, biological product or medical device regulated by FDA must be reviewed, approved and monitored by an institutional review board. An IRB is made up of doctors, researchers and members of the community, who make sure the study is ethical and the rights and welfare of participants are protected.

Relationship to Usual Health Care

While enrolled in a clinical study, participants continue to see their usual health care providers who work with the research team to make sure the study will not conflict with other medications or treatments.

Interested in Participating?

Anyone interested in participating in a clinical study should know as much as possible about the study and feel comfortable asking the research team questions about the study, the related procedures and any expenses. The following questions may be helpful during such a discussion:

  • What is being studied and how long
    will it last?
  • Why do researchers believe the intervention being tested might be effective?
  • What will I have to do and is hospitalization required?
  • Who will know which intervention I receive during the trial? Will I know? Will members of the research team know?
  • How do the possible risks, side effects and benefits of this trial compare with those of my current treatment?
  • What tests and procedures are involved?
  • Who will pay for my participation and will I be reimbursed for other expenses?
  • What type of long-term follow-up care is part of this trial?
  • If I benefit from the intervention, will I be allowed to continue receiving it after the trial ends?
  • Will results of the study be provided to me? Who will oversee my medical care while I am participating in the trial?
  • What happens if I am injured during the study?

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