A study design in which two or more groups subject to different experiences or treatments are compared. The purpose is to make statistical comparisons between two or more groups and demonstrate a causal relationship between the independent variable and outcome of interest.
Between-group designs use a separate sample of individuals for each treatment condition. By comparison, a within- subject design (such as a before-after design) uses the same individuals for each treatment condition.
A research team is interested in understanding the effect of a drug use on learning, and designs a study to compare school grades of a group of students who use the drug with students who do not use it. The two groups are not the same, but if they are similar in every aspect other than drug use, may be compared to evaluate the effects of the drug.
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.