A study design in which the dependent variable (such as a clinical outcome) is measured both before and after an intervention in the same group of individuals. Comparison of the outcomes is made before and after the intervention to assess the effect of treatment.
To evaluate the effectiveness of a immunization program for seniors, researchers sampled a group of seniors and recorded the incidence rates of the disease among the group. After the program has been in place a year, the researchers returned to measure the incidence rates of the disease a second time. The researchers then compared rates of disease before and after implementation of the program to assess the program's effectiveness. Incidence of the disease after implementation of the program is lower than before, the study might find some evidence suggesting that the program has an effect.
Gordis, L. 2000. Epidemiology, 2nd ed., Philadelphia, PA: Elsevier Science.
Marczyk, G.R. et al. 2005. Essentials of Research Design and Methodology, Hoboken, NJ: John Wiley & Sons.
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