ABSTRACT
This unit describes the central features, advantages and disadvantages of experiments, and explores key threats to validity in research. The critical role of random assignment as well as the merits and limitations of control groups and pre-test/post-test designs are considered. Variations such as within-subjects and field experiments are explored, along with a discussion of multi-factorial designs. Threats to validity are presented in a way that emphasizes the advantages and limitations of random assignment in minimizing harms to internal validity, and the way that many threats may affect both internal and external validity. Considerations include ecological validity, mortality, sensitization, regression toward the mean, history, and observer effects.
