Glossary I-R

Below is a glossary of clinical trial terms that are meant to help you to better understand the language of clinical trials.

See more clinical terms:

Informed Consent

The process of learning facts about a clinical trial before deciding whether or not to participate. It is also a continuing process throughout the study to provide information for participants. The study team will explain the details of the study before the participant decides to take part in the clinical trial.

Institutional Review Board (IRB)

A committee of members of varying backgrounds, including representatives from the medical staff, chaplaincy services, nursing, pharmacy, administration, and the community, that ensures a clinical trial is ethical and protects the rights of study participants. The IRB initially approves and periodically reviews the research.

Phase I Trials

Initial studies test an experimental drug or treatment in a small group of people (20-80) for the first time to evaluate its safety, determine a safe dose range, and identify side effects.

Phase II Trials

The experimental study drug or treatment is given to a larger group of people (100-300) to not only see if it is effective but also to further evaluate its safety.

Phase III Trials

The experimental study drug or treatment is given to large groups of people (1000-3000) in order to confirm its effectiveness, monitor side effects, compare it to commonly used treatments, and collect information that will allow the experimental drug or treatment to be used safely.

Phase IV Trials

Post-marketing studies provide additional information such as the drug or treatment risks, benefits and optimal use.


A placebo is an inactive pill, liquid, or powder that has no treatment value. In some studies, the participants in the control group will receive a placebo instead of the experimental drug or treatment. A sick participant will not receive a placebo if there is a known beneficial treatment.

Principal Investigator (Study Doctor)

A medical researcher who is responsible for carrying out a clinical trial's protocol. The study doctor should not be used as the participant’s primary care doctor.


A detailed study plan that is carefully designed to safeguard the health of the participants as well as Definition specific research questions. A protocol describes what types of people may participate in the trial; the schedule of tests, procedures, medications, and dosages; and the length of the study.


A method based on chance which assigns study participants to a treatment group. Randomization reduces the differences among groups by equally dividing the participants with different characteristics among all the trial arms. It is unknown which treatment is better.