Types of Clinical Trials

Clinical research is medical research that is done in human participants after initial (“preclinical”) research has been conducted in a lab setting.1, 2 All research that involves people is called clinical research.3 The purpose of clinical research is to advance medicine.

There are many different types of clinical research, each aimed at advancing medicine in a different way.

What Are the Different Types of Clinical Research?

Clinical research can be divided into different categories based on the questions that the researchers are trying to answer.3


Epidemiology involves studying patterns, causes, and effects of both health and disease among different groups in a population.3, 4 This allows researchers to better understand diseases.


Behavioral research involves studying human behavior and how it is associated with both health and disease.3

Health Services

Health services research involves studying the use and costs of healthcare services and the health outcomes of people that use them.3


Prevention research involves the study of ways to prevent a disorder from developing or returning.3,4Interventions can include medicines, medical devices, vaccines, vitamins or minerals, and behavioral changes.


Screening research involves the study of ways to identify disorders early on, sometimes even before there are any symptoms.2, 3, 4, 5


Diagnostic research involves the study of tests or procedures for diagnosing a disorder or condition.3, 4, 5


Treatment research involves the study of ways to treat a particular disorder.4, 5 Interventions can include medicines, medical devices, procedures, and behavioral changes.3, 4, 5 While some treatments may be new, existing treatments may also be used in new ways. Examples of new ways to use existing treatments include combining them with other medicines or using them to treat a different disorder.3

Quality of Life

Quality of life research, sometimes called supportive care research, involves the study of ways to increase comfort and improve quality of life for people living with long-lasting conditions or illnesses.2, 3, 5 Sometimes this research involves managing the symptoms of a condition or the side effects that can occur from treating the condition.6

What Are the Different Types of Clinical Studies?

Clinical trials and observational studies are the two main types of clinical studies.5

Clinical Trials

Clinical trials are also sometimes called interventional studies.5 Clinical trials help researchers like scientists and doctors test new ways to prevent, screen, diagnose, treat, or manage a medical condition.3. 4. 6 Clinical trials are carefully planned by the researchers.7 Participants in clinical trials are separated into groups and assigned a specific treatment.3, 5 Examples of treatments are medicines, vaccines, medical devices, procedures, or behavioral changes.3, 4, 5 Researchers then use different tests to measure how the treatment affects the participants’ health.5, 7

Clinical trials are often divided into a sequence of four phases.4, 7 Each phase of a trial has a different purpose. The Food and Drug Administration, or FDA, approves advancement to each phase of a clinical trial.5 Clinical trials vary in size and can include people of all ages who are healthy or affected by the condition being studied.4, 7 Well-designed clinical trials are often considered the best way to find out whether a treatment works.3

Clinical Trial Example
One clinical trial aimed to find out if a specific relaxation technique could help reduce anxiety and pain in children having minor surgery.7 The children were separated into two groups: one group used the relaxation technique and the other group received the usual care. The researchers used the data they collected to find out if the relaxation technique reduced anxiety before surgery and pain after surgery any better than the usual care did.

Observational Studies

Some research questions might require a different type of clinical study.3, 8  Observational studies are another type of clinical study that can be used to answer research questions.5, 9 Observational studies help researchers understand a situation by observing people in normal settings.2, 3 Observational studies are different from clinical trials in that participants in observational studies are not assigned to a specific treatment.3, 5, 9 Instead, participants may receive a treatment as part of their usual medical care or be exposed to something in their everyday lives.5, 9 Researchers collect data and identify associations between factors by comparing the changes that they observe over time.2, 3

Researchers may use observational studies instead of randomized controlled clinical trials for different reasons. One prominent reason is that certain interventions may be unethical to test.9 In addition, observational studies are generally much less expensive to run and may be more appropriate for determining effects over a long range of time.10

Observational studies can have a variety of designs.3, 9 Some common types of observational studies include cohort, case-control, and cross-sectional studies.

Cohort Study

In a cohort study, a large group of people is observed over time.3, 9 Some people might naturally develop a disease or condition during this time period. This type of study can help find links between risk factors and disorders.

Cohort Study Example
The Nurses’ Health Study was started in 1976 and is a classic example of a cohort study.11 This study has examined the risk factors for major chronic diseases in more than 280,000 women.

Case-Control Study

Case-control studies consist of people who have a condition or disease and people who don’t.3, 9 This type of study can help find links between disorders and past exposures to risk factors.

Case-Control Study Example
One case-control study evaluated previous use of a pain reliever medicine in individuals who had kidney disease and people who did not have kidney disease.12 The researchers used the data they collected to find out if a pain reliever medicine might be associated with an increased risk of kidney disease.

Cross-Sectional Study

A cross-sectional study is a snapshot of a population at a single moment in time.3, 9 This study design is used to determine the prevalence of a condition or disease, risk factors, and exposures.

Cross-Sectional Study Example
A cross-sectional study has evaluated the association between banana consumption and depressive symptoms in Chinese adults.13

Researchers can use the results of observational studies to gather ideas about the research questions that can be answered in clinical trials.2, 3