WHY DO

RESEARCHERS

DO DIFFERENT KINDS OF

CLINICAL STUDIES?

Clinical research is the study of health and illness in people.

Scientists may have many reasons for doing a clinical study, such as:

• To explore the cause of a disease or a set of symptoms

• To test if a treatment will help with a symptom or condition

• To learn how a certain behavior affects people’s health

Different types of clinical studies are used in different circumstances. Depending on what is known and 

what isn’t, scientists may even study the same research question using different kinds of studies and in 

different groups of people. Here are different types of clinical studies and why they might be used.

Observational Studies

In many studies, researchers do not do experiments 

or test new treatments; they observe. Observational 

studies help researchers understand a situation and 

come up with hypotheses that can be put to the 

test in clinical trials. Observational studies can find 

associations between things but can’t prove that one 

thing causes another. Types include:

Case Study/Case Series

A detailed description of one or 

more patients. By documenting 

new and unusual cases, 

researchers start to generate 

hypotheses about causes or 

risk factors.

Ecological Study

Compares the rate of a 

disease or condition for 

groups of people, such 

as towns in different 

climates or with different 

average incomes.

Cross-Sectional Study

A snapshot of many people 

at one moment in time. 

These studies can show 

how common a condition 

is and help identify factors 

associated with it. 

Case-Control Study

A group of people 

who have a condition 

is compared to a 

control group of people 

who don’t. Possible 

causes or risk factors 

can emerge.

Cohort Study

A large group of people 

is observed over time. 

Some eventually develop 

a disease or condition. 

Researchers can learn 

how often the condition 

occurs and find possible 

causes or risk factors. 

Clinical Trials

In these studies, researchers test new ways 

to prevent, detect, or treat disease. Treatments 

might be new drugs or combinations of drugs, 

new surgical procedures or devices, or new 

ways to use existing treatments. Clinical trials 

can also test other aspects of care, such as 

ways to improve the quality of life for people 

with chronic illnesses. 

A well-designed clinical trial is the 

gold standard for proving that a 

treatment or medical approach 

works, but clinical trials can’t 

always be used. For example, 

scientists can’t randomly assign 

people to live in different places, 

or ask people to start smoking or 

eating an unhealthy diet. Clinical 

trials are conducted in phases:

Phase I

• Purpose: Find out whether a medical 

approach (e.g., drug, diagnostic test, 

device) is safe, identify side effects, 

and figure out appropriate doses.

• Number of people: Typically fewer 

than 100

Phase II

• Purpose: Start testing whether a 

medical approach works. Continue 

monitoring for side effects; get 

information that goes into designing a 

large, phase III trial. 

• Number of people: Typically 100-300

Phase III

• Purpose: Prove whether a medical 

approach works; continue monitoring 

side effects. 

• Number of people: As many as 

needed or able to enroll—can 

be 1,000 or more

Phase IV

• Purpose: When a medical approach 

is being marketed, continue gathering 

information on its effects. 

• Number of people: Thousands

How good are these kinds 

of studies at showing cause 

and effect?

The strength of a study depends on its size 

and design. New results may confirm earlier 

findings, contradict them, or add new aspects 

to scientists’ understanding. In the end, cause 

and effect are usually hard to establish without 

a well-designed clinical trial.

Most 

Effective

Least 

Effective

Clinical Trial

Cross-Sectional Study

Ecological Study

Case Study/Series

Case-Control Study/Cohort Study

What can I do to help?

You’ve begun! Learning about what results mean will 

help you make good choices with your health care 

provider.

You could also consider volunteering either as a 

healthy volunteer or as a participant who has a 

particular disease or condition.

For more information about clinical trials: 

ClinicalResearchTrials.nih.gov

CRAY@mail.nih.gov

Produced by the National Institutes of Health, the largest source of 

public funding for medical research in the world. NIH’s mission is 

to seek fundamental knowledge about the nature and behavior of 

living systems and the application of that knowledge to enhance 

health, lengthen life, and reduce illness and disability.