Basics Print Observational Study for Medical Research Medically reviewed by facebook linkedin Medically reviewed by Richard N. Fogoros, MD on April 11, 2016 Richard N. Fogoros, MD, is a retired professor of medicine and board-certified internal medicine physician and cardiologist. He is Verywell's Senior Medical Advisor. Learn about our Medical Review Board Richard N. Fogoros, MD Written by Written by Shereen Lehman, MS Shereen Lehman, MS, is a healthcare journalist and member of the Academy of Nutrition and Dietetics and the American Society for Nutrition. Learn about our editorial policy Shereen Lehman, MS Updated on June 27, 2019 Caiaimage/Martin Barraud / Getty Images More in Basics Hot Topics Food Safety An observational study is an epidemiological research study that doesn't include some intervention or experiment. Subjects are studied under natural living conditions. Scientists use observational studies to hunt for possible relationships between exposures and outcomes. They're used for several areas of health, and many of the studies you hear about on TV or read about in websites, magazines, and newspapers are observational studies. An 'outcome' is usually a disease or health problem of some sort. The scientists use information from things like surveys and medical records to see if certain subjects have something or things in common. These things are called 'exposures.' When a sufficient amount of research indicates an exposure increases the risk of the outcome, then the exposure is known as a risk factor. An example of exposure is a risk factor would be eating large amounts of processed meat, which is a risk for developing certain types of cancer. Sometimes exposures can be protective, like eating a diet rich in fruits and vegetables, which appears to reduce the risk of heart disease. Types Most observational studies fall into one of three categories, case/control studies, cohort studies/ and cross-sectional studies. Case/control studies start with a group of subjects who have the outcome being studied (the cases) and another group of those who don't have it (the controls). Scientists then look back in time to see if the cases have any exposures in common that the controls do not or vice versa. Case/control studies are called retrospective studies because they start with the outcome and look backward in time to determine the cause or risk. Cohort studies take a large number of subjects and group them by exposures, then follow them for some time (often years and decades) to see who develops the outcome they're studying. Again, scientists are looking to see if members of any of the groups have exposures in common. Scientists may have to wait years for the results unless they use a large ongoing study, such as the National Health and Nutrition Examinations Survey (NHANES). Thousands of people participate every year by answering questions and undergoing physical examinations. Scientists sift through information gathered from NHANES to look for all kinds of connections between foods, dietary supplements, and health. For example, NHANES information was used to determine that folate (a B-complex vitamin) deficiency can lead to birth defects. Cohort studies start before anyone has the outcome and look forward in time, so they're called prospective. Finally, cross-sectional studies don't look forward or backward; they only look at what's going on at one particular time. Scientists can determine how many people have the outcome of interest and attempt to look for exposures, but without a longer time-frame, it's difficult to know for sure. Strengths and Weaknesses Observational studies are extensive, often with thousands of participants, which gives strength to the results, but they usually can't determine any cause. Since subjects live normally, there are usually too many possible exposures that can confound the results. For example in many dietary studies, people who eat large amounts of red meat also tend to smoke, eat less fiber, and exercise less than average. Subjects who eat the least amount of red meat also exercise more, eat more fruits and vegetables than average, and rarely smoke. Scientists use various statistical techniques to remove potential confounding factors, but sometimes the results are still a little murky. Sometimes the results of observational studies lead to randomized controlled trials (RCT), which are interventional, or experimental, studies and thought to provide the best research evidence. That's because subjects are randomized into treatment and control groups, which reduces the effect of confounding factors. Why is Sampling Used in Research? Was this page helpful? Thanks for your feedback! Looking to lose weight? Our nutrition guide can help you get on the right track. Sign up and get it free! Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit Article Sources Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Centers for Disease Control and Prevention. "About the National Health and Nutrition Examination Survey." http://www.cdc.gov/nchs/nhanes/about_nhanes.htm. Jepsen P, Johnsen P, Gillman MW, Sørensen HT. "Interpretation of observational studies." Heart. 2004 August; 90(8): 956-960. http://heart.bmj.com/content/90/8/956 Continue Reading