The NIH News in Health
skip navigation
June 2007
(PDF—686 kb)  

Understanding Clinical Research
Learn the Basics

Man with seeing eye dog walking arm in arm with a doctor
You hear about the results of clinical research all the time.  You’ve probably made a few changes to your behavior—what you eat or how much you exercise—based on some study.  Whether you want to understand how to put those news reports in perspective or are thinking of participating in a clinical study yourself, it’s time you learned more about clinical research.

There are two general types of clinical research studies.  In an observational study, scientists observe people to learn more about the cause or progression of a disease or condition.  They might compare a group with a particular condition to another similar group without the condition to figure out what factors play a role in the disorder.  Or they might select participants based on their exposure, say, to a particular pollutant, and then try to find a similar group of people who haven’t been exposed.  By comparing the two groups, they might discover whether the factor in question is causing any health problems.

In an intervention study, researchers test a particular treatment.  The best-known type of intervention study, the randomized clinical trial, is considered the gold standard.  People are assigned to 2 or more study groups by chance (randomly).  The different groups receive different treatments.  One, the control or comparison group, receives a sham treatment or a pill that looks just like the drug being tested but actually does nothing (called a placebo).  Only the pharmacist knows who is getting which medication so that observations by the research team won’t be biased.

Comparing treatment groups to control groups is the best way to see if a treatment is really effective.  But that’s not always possible.  Placebos can’t be used if a patient would be put at risk by not having effective therapy.  In these situations, studies compare the experimental therapy with an approved one.

You may hear about clinical trial “phases.”  Each phase has a different purpose and helps researchers answer different questions.  In phase I trials, researchers test an experimental drug or treatment in a small group of people (20–80) for the first time.  The purpose is to evaluate its safety and identify side effects, not to see if it’s effective.

In phase II trials, the experimental drug or treatment is given to a larger group of people (100–300) to start testing its effectiveness and further evaluate its safety.  In phase III trials, the experimental drug or treatment is administered to large groups of people (1,000–3,000) to confirm its effectiveness, monitor side effects, compare it with other treatments and collect information about how to use the experimental treatment safely.

After a treatment is licensed (approved by the U.S. Food and Drug Administration), researchers track its safety in phase IV trials, seeking more information about its risks, benefits and optimal use.  Large groups of participants are needed for these long–term studies to detect any unexpected side effects that might occur in a small percentage of people.

Wise Choices iconWise Choices
Questioning Research Study Results

  • Was the study done in the laboratory, in animals or in people?
  • Does the study include enough people like you?
  • What kind of trial was it?  A randomized clinical trial involving thousands of people gives scientists the most reliable results.
  • Where was the research done and who paid for it?
  • If a new treatment was being tested, were there side effects?
  • Who is reporting the results?  Is it from a source you can trust?
  • If you’re not sure what a finding means for you, ask your health care provider.

Links iconWeb Sites


to top    
NIH logo National Institutes of Health (NIH)
9000 Rockville Pike
Bethesda, Maryland 20892
DHHS logo Department of Health and
Human Services
  Office of Communications and
Public Liaison