A Guide to Understanding Clinical Trial Terminology

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Clinical trials have come garnered much attention during the pandemic and it has raised many questions regarding the terminology used. Clinical trials are robust scientific experiments that test new medical treatments on humans to prove their safety and efficacy. This article aims to provide a brief overview of the terminology used in clinical trials.

What are clinical trials?

Clinical trials involve a series of studies where a new drug or treatment is investigated and compared to the standard of treatment currently available. After a new drug has undergone preclinical testing or lab testing, it then applies for authorization to be tested on humans to fully understand the benefits and risks of the drug.

This is a necessary process as these documents and results are then submitted to the national health authority such as the U.S. Food and Drug Administration (FDA) which analyses the data and makes a decision on whether the drug is approved for use.

Clinical Trial

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The phases of a clinical trial?

Clinical trials have four phases and approval is required for every phase separately.

Phase I: This is the non-therapeutic phase as the aim of this is to test that the new drug is safe and tolerable in humans. This usually involves a small group of participants.

Phase II: This phase is to gauge the preliminary effectiveness of the drug and the range of effective doses that can be used. It is also used to provide a comparison to the standard treatment or ‘market leader’ and the placebo. The participants are usually patients with a defined disease for which the drug being tested is indicated.

Phase III: This is the pivotal phase of the clinical trials which is the most expensive and time-consuming. The aim of this is to confirm the effectiveness observed in phase II and to gather additional data on the benefits and risks of the drug. It also confirms efficacy to the ‘market leader’ or standard drug treatment.

The results of these three phases are sent to the health authority that will then either grant approval or reject the drug. If approved, then phase IV of the clinical trial is initiated.

Phase IV: this is the post-marketing and surveillance stage after the approval and the aim of this is to identify any long-term effects of the drug in a population.

Principal investigator?

Also known as a medical researcher; they are in charge of overseeing the clinical trial through different phases and authorizations.

What is an Inclusion/Exclusion Criteria?

These are the standard that determines whether a volunteer can participate in clinical trials. These factors are determined based on the drug that is being tested and usually include age, gender, the type and stage of disease, and other medical conditions. The aim of this is to identify appropriate participants to keep them safe.

Interventional and observational study?

In an interventional study, something is given to the participants as a treatment, such as a drug, and the effects on the group are analyzed and recorded.

In an observational study, treatment or drug is not given but rather a group of participants are studied to understand how risk factors and diseases are related.

What is a control group?

In a clinical trial, one group of patients will receive an intervention and another group will not; this is to compare the outcomes between the groups. The group that doesn’t receive an intervention is known as the control group and is the standard to which the experimental observations are evaluated. The control group can either be given a standard treatment or a placebo.

What is Placebo and Placebo Effect?

Placebo is an inactive drug that has no therapeutic effect; it is physically similar to the drug being tested. The aim is to ensure that the participants are unaware if they are taking the dummy treatment or the drug being tested so that a bias or pre-conceived perception is avoided. Any difference in outcome will only be attributed to the drug rather than a placebo effect.

The placebo effect is a physical or emotional change occurring after a substance is taken and not as a result of the specific properties of a drug. The change may be beneficial and reflects the expectation of the participant or the administrator.

Placebo Concept

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What is a double-blind randomized control trial?

Randomized controlled trials are interventional studies that study the cause-effect relationship of a drug being studied. Participants selected according to the eligibility criteria are randomly assigned to a group that is either a control group or the intervention group. Random assignment is to minimize bias and balance individual characteristics so any differences between the groups are attributed to the intervention.

A study is considered double-blind when neither the patient nor the researchers involved are aware which group is the control group and which is the treatment group. This is to avoid aims and expectations that may influence the outcome. This is considered the gold standard of studies as it is tightly controlled and external variables are significantly reduced or balanced between the control and treatment group.


  • Glossary of Common Site Terms - ClinicalTrials.gov. (2021). Clinicaltrials.Gov. https://clinicaltrials.gov/ct2/about-studies/glossary
  • Glossary of Terms Used in Clinical Trials. (2010). University of Nottingham. https://www.nottingham.ac.uk/research/groups/cebd/documents/patientscarers/glossaryoftermsusedinclinicaltrials.pdf

Further Reading

Last Updated: Mar 31, 2022

Storay Amiri

Written by

Storay Amiri

Storay attended the University College of London (UCL) to Study Master of Pharmacy and graduated in 2021 with First Class Honors. A four-year integrated master degree with a detailed focus on all parts of medication, from chemistry to body therapeutics, pharmacology, pharmaceutics, and more.    


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