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Accepts healthy volunteers
A type of eligibility criteria that indicates whether people who do not have the condition/disease being studied can participate in that clinical study.

Adverse event
An unfavorable change in the health of a participant, including abnormal laboratory findings, that happens during and after a clinical trial or within a certain amount of time after the study has ended. This change may or may not be caused by the intervention/treatment being studied.

A group or subgroup of participants in a clinical trial that receives a specific intervention/treatment, or no intervention, according to the trial's protocol.


Clinical study/trial
A research study involving human volunteers (also called participants) that is intended to add to medical knowledge. There are two types of clinical studies: interventional studies (also called clinical trials) and observational studies. identifier (NCT number)
The unique identification code given to each clinical study upon registration at The format is "NCT" followed by an 8-digit number (for example, NCT00000000).


Data Monitoring Committee (DMC)
A group of independent scientists who monitor the safety and scientific integrity of a clinical trial. The DMC can recommend to the sponsor that the trial be stopped if it is not effective, is harming participants, or is unlikely to serve its scientific purpose. Members are chosen based on the scientific skills and knowledge needed to monitor the particular trial. Also called a data safety and monitoring board, or DSMB.


Eligibility criteria
The key requirements that people who want to participate in a clinical study must meet or the characteristics they must have. Eligibility criteria consist of both inclusion criteria (which are required for a person to participate in the study) and exclusion criteria (which prevent a person from participating). Types of eligibility criteria include whether a study accepts healthy volunteers, has age or age group requirements, or is limited by gender.

The number of participants in a clinical study. The "estimated" enrollment is the target number of participants that the researchers need for the study.

Exclusion criteria
A type of eligibility criteria. These are reasons that a person is not allowed to participate in a clinical study.

Expanded access
A way for patients with serious diseases or conditions who cannot participate in a clinical trial to gain access to a medical product that has not been approved by the U.S. Food and Drug Administration (FDA). Also called compassionate use. There are different expanded access types.


Inclusion criteria
A type of eligibility criteria. These are the reasons that a person is allowed to participate in a clinical study.

Informed consent
A process used by researchers to communicate to potential and enrolled participants the risks and potential benefits of participating in a clinical study. For more information, see Participating in Studies on this site.

Informed consent form (ICF)
The document used in the informed consent or process.

A researcher involved in a clinical study. Related terms include site principal investigator, site sub-investigator, study chair, study director, and study principal investigator.


Participant flow

A summary of the progress of participants through each stage of a clinical study, by study arm or group/cohort. This includes the number of participants who started, completed, and dropped out of the study.


The stage of a clinical trial studying a drug or biological product, based on definitions developed by the U.S. Food and Drug Administration (FDA). The phase is based on the study's objective, the number of participants, and other characteristics. There are 4 stages: Phase 1,  Phase 2, Phase 3, and Phase 4. 

Phase 1

A phase of research to describe clinical trials that focus on the safety of a drug. They are usually conducted with healthy volunteers, and the goal is to determine the drug's most frequent and serious adverse events and, often, how the drug is broken down and excreted by the body. These trials usually involve a small number of participants.

Phase 2

A phase of research to describe clinical trials that gather preliminary data on whether a drug works in people who have a certain condition/disease (that is, the drug's effectiveness). For example, participants receiving the drug may be compared to similar participants receiving a different treatment, usually an inactive substance (called a placebo) or a different drug. Safety continues to be evaluated, and adverse events are monitored.

Phase 3

A phase of research to describe clinical trials that gather more information about a drug's safety and effectiveness by studying different populations and different dosages and by using the drug in combination with other drugs. These studies typically involve more participants.

Phase 4

A phase of research to describe clinical trials occurring after the FDA has approved a drug for marketing. They include post-market requirement and commitment studies that are required of or agreed to by the study sponsor. These trials gather additional information about a drug's safety, efficacy, or optimal use.


An inactive substance or treatment that looks the same as, and is given in the same way as, an active drug or intervention/treatment being studied.

Principal investigator (PI)

Primary healthcare provider at a site location who is responsible for the conduct of a clinical trial.


The written description of a clinical study. It includes the study's objectives, design, and methods. It may also include relevant scientific background and statistical information.


Serious adverse event
An adverse event that results in death, is life-threatening, requires inpatient hospitalization or extends a current hospital stay, results in an ongoing or significant incapacity or interferes substantially with normal life functions, or causes a congenital anomaly or birth defect. Medical events that do not result in death, are not life-threatening, or do not require hospitalization may be considered serious adverse events if they put the participant in danger or require medical or surgical intervention to prevent one of the results listed above.

The organization or person who initiates the study and who has authority and control over the study.

Study design
The investigative methods and strategies used in the clinical study.

Study results
A study record that includes the summary results posted in the results database. Summary results information includes participant flow, baseline characteristics, outcome measures, and adverse events (including serious adverse events).


Treatment arm

An arm type in which a group of participants receives the intervention/treatment that is the focus of the clinical trial.