Glossary of Terms: Clinical Trials

Clinical trial site

A clinical trial study can have multiple sites, including physical locations or virtual locations (online, by phone). These locations may even be in different states. A study may choose to be in several locations because of the resources to conduct the study (the trial needs to have clinicians and professionals that administer treatments or do assessments) and because they want to reach a diverse pool of participants.

Control group

In a clinical trial, a treatment will be tested to see if it is effective as a treatment for the disease/condition or if it is better than the standard treatment. Participants will be separated into groups: one that is given the experimental treatment and the other that is not so they can serve as a comparison group (control group). The control group may get the standard treatment, a placebo, or no treatment at all.

Exclusion criteria

Exclusion criteria differ for each trial and are things you must not be or have in order to participate in that particular clinical trial. Meeting/having one or more of the exclusion criteria means that you definitely cannot participate in the trial. For example, a study may exclude participants who have taken a specific medication in the past. The reason for an exclusion criteria like this is that taking the medication may have changed your body chemistry in a way that the researchers can’t know for sure or know how it will affect the impact of the experimental treatment. Other examples of exclusion criteria are specific ages or pregnancy.

Informed consent

It is important that people considering a clinical trial know that their participation is voluntary, know all the details of the study, and have an opportunity to ask questions before agreeing to participate in a clinical trial. Potential participants will learn all of this in the mandatory informed consent process.

Institutional Review Board (IRB)

An institutional review board (IRB) is a group of independent doctors, researchers, and members of the community who are trained to review research studies to make sure the study is designed ethically and that the researchers communicate clearly and honestly about the risks and benefits about participating. Most importantly, the IRB ensures that the rights and welfare of the participants are protected. Learn more about your rights as a research participant.

Interventional study

In an interventional study, a group of participants are assigned to receive an experimental treatment and a group doesn’t get the experimental treatment. The goal of this type of study is to see how the health outcomes from the experimental treatment compare to the health outcomes from standard treatment, a placebo, or no treatment at all.


Masking (or blinding) occurs when the participant, clinical trial doctors, and/or researchers involved do not know whether an individual received the placebo, standard treatment, or the drug/device intervention. This helps to prevent the “placebo effect” from influencing the results of the experiment and is again an attempt to remove a layer of bias from the study. Sometimes, participants can feel better just because they think they are taking the study drug–this is the placebo effect. When the time comes to inspect the results and if meaningful differences are found in the clinical trial results, researchers can be more confident that the differences are a result of the drug’s impact despite (or not due to) the placebo effect.

Observational study

In an observational study, participants are not asked to change their normal routine. They can continue taking the medication and doing management practices they have been doing before enrolling. The investigator of this study will then observe the participant through various assessments, like surveys or medical tests. A health registry where patients input information themselves, like NEA’s EczemaWise app platform that collects real-world patient experience data, is a type of observational study.


An inactive treatment that looks like the treatment being studied, and is provided in the same manner as the treatment being studied, but has no known therapeutic benefits. This is to make sure differences seen between the treatment group and the non-treatment group (control group) are really due to the characteristics of the drug, not the state of mind of the participant. A placebo effect can occur when an improvement of symptoms is observed in the clinical trial, despite not receiving the test drug.

Principal investigator (PI)

The principal investigator is the person who leads the study and takes full responsibility for the study’s execution and reporting.

Rescue therapy

Rescue therapy is an option that some clinical trials offer where the clinical trial will put the participant on a medication to help with unexpected flares or other symptoms that are exacerbated by being on a new treatment or no treatment. Unfortunately, this means that when it comes time for the researchers to inspect the data, those that went on rescue therapy will show reduced observed treatment effect. This is something researchers take into account when doing their analysis and discussion of the results.

Study benefits

There are always risks and benefits to weigh in deciding whether to participate in a clinical trial. The benefits that participants generally think of are the benefits of receiving the experimental treatment that may work better than their current treatment, but there are other benefits to weigh, such as increased awareness about your own condition by being a part of the research process or the satisfaction with the knowledge that you are contributing to research that may ultimately benefit more people with your condition.

Study risks

There are always risks and benefits to weigh in deciding whether to participate in a clinical trial. The risks that participants are often most concerned about are side effects from an experimental drug, but there are also other risks such as taking more time to go to the study site to receive care and follow-up, tests and procedures you may not normally receive, or a more complicated treatment regimen to follow.

Treatment arm

The group of clinical trial participants who receive the test drug/device intervention. Clinical trials also have a ‘control arm’ of individuals who did not receive the test drug/device. By comparing the results of the control to the treatment arm, investigators can determine the efficacy and safety of the test drug/device.

Treatment randomization

Treatment randomization is when the different therapies being tested in a clinical trial get assigned to the participant randomly (by chance), rather than by choice. This is done to balance the numbers of individuals receiving different therapies as well as eliminate a key bias that can potentially influence the objectivity of the study. Clinical trial investigators don’t know which treatment participants have received until after the trial is completed.

Washout period

The washout period is a period of time where a participant must stop taking a specific medication before beginning the clinical trial in order to make sure it is completely flushed from the body before a new treatment or assessment is given.

View a more comprehensive list of clinical trials terms.

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