Clinical Trial Terminology

There are several terms that are commonly used when discussing clinical trials. Here are some that come up often:1,2

Adverse event: an undesirable experience associated with the use of a medical product in a participant.

Arm: a grouping of study participants who receive a specific intervention, or no intervention, according to the study protocol.

Baseline characteristics: data collected at the beginning of the study for each participant, including demographics like age, gender, and race. Data for study-specific characteristics like prior treatment, weight, etc., will also be collected.

ClinicalTrials.gov identifier (NCT number): the unique identification code assigned to each clinical trial that is registered on ClinicalTrials.gov.

Controlled Trial: A type of clinical trial that examines an intervention compared with a standard (called the control).


Double Blind Trial: A clinical trial where at least 2 parties involved, usually the patient and the investigator, do not know what intervention the patient is receiving. This is to avoid potential bias.

Exclusion Criteria: Factors that would prevent a person from participating in a study.

FDA: Food and Drug Administration.

Inclusion Criteria: Factors that allow a person to participate in a study.

Intervention: The focus of the clinical study. This could be a drug, a medical device, a medical procedure, other investigational products, as well as surveys, education, or interviews.

Investigator: A person who is conducting research of a clinical study.

Open Label: A term describing a clinical trial where blinding is not used. All parties involved know which study participants have been assigned to which interventions.

Outcome Measure: A measurement that is outlined in the study protocol that is used to determine the effect of the intervention.

Placebo: A substance that does not contain active ingredients and is designed to appear the same as the intervention being examined.

Primary Outcome Measure: The main outcome measure that is evaluating the effect of the study intervention.

Written by: Anna Nicholson | Last Reviewed: January 2016.