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Frequent words defined

The following section was created using several sources of information considered as reliable and transferable for CDG. This section provides definitions for words and terms you may want to know. Many of these words are also used by clinical researchers and others in the same or a similar manner. They were compiled specially to help those who are searching for clinical trials on You can complement this information with the Glossary of clinical research created by Wikipedia.

For an Introduction to Clinical Research check the following video:

ClinicalTrials gov Identifier

The unique identification code given to each clinical study upon registration at The format is "NCT" followed by an 8-digit number (for example, NCT00000419).

Visit for more information.

Adverse event (AE)

Any medical event (including intercurrent diseases and accidents) that occurs under treatment with a medicinal product that does not necessarily have a causal relationship with the treatment.

Visit Caduceus for more information.


In a clinical trial, an ‘arm’ is an assigned group, for example a trial may include a placebo arm and an investigational intervention arm. Some types of trial design include more than two arms. Participants are usually randomly assigned to these groups.

Visit Global Health Trials for more information.

Eligibility Criteria

The requirements that people who want to participate in a clinical study must meet. Eligibility Criteria include both inclusion criteria and exclusion criteria and are defined in the protocol.

Visit Novartis for more definitions.

Exclusion criteria and Inclusion criteria

Powtoon created an explanatory and animated video that explains easily both concepts:

The key requirements that people who want to participate in a clinical study must meet or the characteristics they must have. Exclusion criteria are reasons that a person is not allowed to participate in a clinical study. Inclusion criteria are the reasons that a person is allowed to participate in a clinical study.

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In clinical trials, an event or outcome that can be measured objectively to determine whether the intervention being studied is beneficial. The endpoints of a clinical trial are usually included in the study objectives. Some examples of endpoints are survival, improvements in quality of life, relief of symptoms, and disappearance of the tumor.
Visit Mary Crowley for more definitions.

Expanded Access

Also called “compassionate use,” provides a pathway for patients to gain access to investigational drugs, biologics and medical devices for serious diseases or conditions. Investigational drugs and devices have not yet been approved by the FDA and they have not been proven to be safe and effective. Therefore, they may be effective in the treatment of a condition, or they may not. It is important to remember that the drug/biologic/medical device may have unexpected serious side effects and that patients need to consider all the possible risks when seeking access to an investigational medical product.

To gain access to an investigational medical product outside of a clinical trial, the sponsors must decide whether to make their experimental medical product available to patients via expanded access.  FDA regulations specify two groups of people eligible for expanded access:

those with life-threatening diseases or conditions for which “there is a reasonable likelihood that death will occur within a matter of months or in which premature death is likely without early treatment” those with serious diseases or conditions that have a “substantial impact on day-to-day functioning”

In most cases, patients who seek compassionate use must have exhausted all approved therapies for their condition, and be unable to enroll in a clinical trial.

Visit FDA for more definitions.

Informed Consent

Informed consent explains risks and potential benefits about a clinical trial before someone decides whether to participate.
Visit NIH for more information.


A process or action that is the focus of a clinical study. This can include giving participants drugs, medical devices, procedures, vaccines, and other products that are either investigational or already available. Interventions can also include non-invasive approaches such as surveys, education, and interviews.

Visit NIHR for more definitions.

Interventional study

A clinical study in which patients are assigned to receive one or more interventions (or no intervention) so that researchers can evaluate the effects of the interventions on biomedical or health related outcomes. The assignments are determined by the study protocol. Participants may receive diagnostic, therapeutic, or other types of interventions.

Visit NIHR for more definitions.

Observational study

An observational study investigates health outcomes amongst groups of people in the course of their everyday life at home, work, or the doctor’s office, where assignment of treatments or other procedures is as part of their regular medical care (not assigned by an investigator). 

Visit Novartis for more definitions.

Outcomes measures

An outcome is the measurable event for trial endpoints, but is usually more general than an endpoint; for example, an endpoint may look at the efficacy of an intervention, while outcomes will look at other aspects such as whether the intervention improves quality of life, works more quickly than existing medications, is more practical, and so on.

  • Primary Outcomes measures - In a clinical study's protocol, the planned outcome measure that is the most important for evaluating the effect of an intervention/treatment. Most clinical studies have one primary outcome measure, but some have more than one.
  • Secondary Outcomes measures - In a clinical study's protocol, a planned outcome measure that is not as important as the primary outcome measure for evaluating the effect of an intervention but is still of interest. Most clinical studies have more than one secondary outcome measure.

For example where improving survival is the primary objective then the outcome is mortality with the trial endpoint being death. Or in a pain study the patient reported measurements are the outcome and the endpoint is pain.

Visit Global Health Trials and for more information.

Principal investigator

The clinical trial doctor - a highly qualified physician who carries out the research and interacts with the patients. All clinical trial doctors also have additional training in clinical trials and research.

Visit Clinical Trials and Me for more definitions.

Recruitment Status

  • Not yet recruiting: The study has not started recruiting participants.
  • Recruiting: The study is currently recruiting participants.
  • Enrolling by invitation: The study is selecting its participants from a population, or group of people, decided on by the researchers in advance. These studies are not open to everyone who meets the eligibility criteria but only to people in that particular population, who are specifically invited to participate.
  • Active, not recruiting: The study is ongoing, and participants are receiving an intervention or being examined, but potential participants are not currently being recruited or enrolled.
  • Suspended: The study has stopped early but may start again.
  • Terminated: The study has stopped early and will not start again. Participants are no longer being examined or treated.
  • Completed: The study has ended normally, and participants are no longer being examined or treated (that is, the last participant's last visit has occurred).
  • Withdrawn: The study stopped early, before enrolling its first participant.
  • Unknown: A study on whose last known status was recruiting; not yet recruiting; or active, not recruiting but that has passed its completion date, and the status has not been last verified within the past 2 years.

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An individual, company, institution or organization taking responsibility for initiation, management, and financing of a clinical study.

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Indicates the current recruitment status (it may be Not yet recruiting, Recruiting, Enrolling by invitation, Active not recruiting, Suspended, Terminated, Completed, Withdrawn or Unknown)or the expanded access status (it may be Available, No longer available, Temporarily not available or Approved for marketing).

Visit for more information.

Study type

  • Diagnostic trials determine better tests or procedures for diagnosing a particular disease or condition.
  • Natural history studies provide valuable information about how disease and health progress.
  • Prevention trials look for better ways to prevent a disease in people who have never had the disease or to prevent the disease from returning.
  • Quality of life trials (or supportive care trials) explore and measure ways to improve the comfort and quality of life of people with a chronic illness.
  • Screening trials test the best way to detect certain diseases or health conditions.
  • Treatment trials test new treatments, new combinations of drugs, or new approaches to surgery or radiation therapy.

Visit National Institutes of Health for more information.

Trials Phases

For faster explanation watch the National Cancer Institute video on the phases of clinical trials below. 

Phase I

Phase I is the first phase in a trial of an investigational drug or vaccine in humans. Prior to phase one, researchers will have considered the dosing requirements through testing with animals or on tissues. Phase I usually involves giving different doses to different participants to look for any short-term side-effects or toxicity and evaluate the metabolism and pharmacologic action of the drug or vaccine with humans specifically. Typically phase I trials involve a small number of healthy volunteers compared with the following phases.

Phase II 

Phase II trials involve a controlled (comparative) study of the intervention to determine its effectiveness with patients. This phase also aims at determining the safety, dosage, side effects and risks of the medication. Typically, phase II studies involve around 100-300 patients.

Phase III

Phase III trials are intended to gain further insight into the safety and efficacy of an intervention in the population for which its use is intended, and therefore this phase should create analyzable data about the risk-benefit ratio of the drug. The data from this phase is then used as the basis for regulatory approval of the drug and subsequent labelling claims.

Phase IV

A Phase IV study may also be known as a surveillance or post-marketing study, because it occurs once the drug has been approved by the regulatory authorities and is being marketed. This phase is useful for identifying any effects of the drug which occur in a minority of users, and also provides additional information on efficacy and safety.

Visit Global Health Trials for more information.


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Bruna Henriques from Sci and Volunteer Program Nova School of Science and Technology 2021.


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Page modified at Wednesday, May 12, 2021 - 09:56