Health Services Research (HSR) Methods
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Quasi-Experimental Study
Synonyms: Quasi-Experimental Approach; Quasi-Experimental Design
Related Terms: Time Series Design

A study design in which researchers manipulate an active independent variable but do not have full control over the allocation or timing of the intervention. Quasi-Experimental designs are often used when it is not possible to conduct a true experiment with complete random assignment, as is often the case in policy or real-life settings.

A researcher is interested in knowing whether a smoking ban is effective in lowering the prevalence of lung cancer. Because the researchers can not conduct a true experiment by randomly implementing smoking bans, they collect data on two cities.  In city A, a smoking ban was implemented five years ago; in city B, no smoking ban has been enacted.

A group of individuals is randomly selected in each city.  Individuals in city A with the smoking ban serve as the experimental group while those in city B are the control group.  At the conclusion of the study, the prevalence rate of lung cancer is compared between the two groups.
The study is called a quasi-experiment because the researcher does not have full control over who receives the intervention.  

Further Reading
Last, J. 1983. A Dictionary of Epidemiology, New York, NY: Oxford Medical Publications.

Marczyk, G.R. et al. 2005. Essentials of Research Design and Methodology, Hoboken, NJ: John Wiley & Sons.

Gliner, J.A. and G.A. Morgan. 2000. Research Methods in Applied Settings, Mahwah, NJ: Lawrence Erlbaum Associates.

Campbell, D. and J. Stanley. 1963. Experimental and Quasi-Experimental Designs for Research, Boston, MA: Houghton Mifflin Company.