StATS: What is an observational design?

A research design where the allocation of treatment is not fully under the control of the researcher. When the patients and/or their doctors chose which treatment they desire, you have an observational design. You can also have an observational design when the groups being studied are intact prior to the start of the experiment. Finally, when researchers are examining an exposure, such as smoking or air pollution, it is often impractical and unethical to assign subjects. Studies of exposure are often observational designs. Observational studies are more likely to require difficult statistical adjustments because of the potential for large covariate imbalances. Here are two examples of observational designs:

• In Cardo (1997), 33 health care workers who became seropositive to HIV after percutaneous exposure to HIV-infected blood were compared to 665 health care workers with similar exposure who did not become seropositive. Since the researchers did not control who became seropositive, this is an observational study.
• In Hu (1997), 80,082 women between the ages of 34 and 59 years were followed for 14 years to look for instances of non-fatal myocardial infarction or death from coronary heart disease. These women were divided into low, intermediate, and high groups on the basis of their consumption of dietary fat. Since the women themselves controlled their diets, rather than having a diet imposed on them by the researchers, this represents an observational design.

This page was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Definitions, Category: Research designs.