Читать книгу Applied Biostatistics for the Health Sciences - Richard J. Rossi - Страница 28

When two or more subpopulations or treatments are to be compared in a biomedical research study, one of the most important aspects of the research protocol is whether the researchers can assign the units to the subpopulations or treatment groups that are being compared. When the researchers control the assignment of the units to the different treatments that are being compared, the study is called an experiment, and when units come to the researchers already assigned to the subpopulations or treatment groups, the study is called an observational study. Thus, in an experiment the researcher has the ability to assign the units to the groups that are being compared, while in an observational study the units come to the researcher already assigned to the groups.

One of the main reasons an observational study is used instead of an experiment in a biomedical research study is that it would be unethical to assign some subjects to a treatment that is known to be harmful and the remaining subjects to a treatment that is not harmful. For example, in a prospective 30-year study of the effects of smoking cigarettes, it would be unethical to assign some subjects to be smokers and others to be non-smokers.

For ethical reasons, observational studies are often used in epidemiological studies designed to investigate the risk factors associated with a disease. Also, a retrospective study is always an observational study because it looks backward in time and the units have already been assigned to the groups being compared. On the other hand, a prospective study and a clinical trial can be run as either experiments or observational studies depending on whether it is possible for the researcher to assign the units to the groups.